Change shutdown_test.cc to use credential provider API to test against different credential types.
Change shutdown_test.cc to use credential provider API to test against
different credential types.

wcevans/grpc,murgatroid99/grpc,ctiller/grpc,PeterFaiman/ruby-grpc-minimal,muxi/grpc,infinit/grpc,hstefan/grpc,yang-g/grpc,carl-mastrangelo/grpc,matt-kwong/grpc,apolcyn/grpc,dgquintas/grpc,mehrdada/grpc,MakMukhi/grpc,royalharsh/grpc,donnadionne/grpc,infinit/grpc,LuminateWireless/grpc,kriswuollett/grpc,grpc/grpc,sreecha/grpc,vsco/grpc,LuminateWireless/grpc,fuchsia-mirror/third_party-grpc,jboeuf/grpc,dklempner/grpc,deepaklukose/grpc,philcleveland/grpc,MakMukhi/grpc,daniel-j-born/grpc,yugui/grpc,kpayson64/grpc,chrisdunelm/grpc,stanley-cheung/grpc,quizlet/grpc,MakMukhi/grpc,Crevil/grpc,muxi/grpc,apolcyn/grpc,carl-mastrangelo/grpc,philcleveland/grpc,carl-mastrangelo/grpc,LuminateWireless/grpc,muxi/grpc,grani/grpc,7anner/grpc,msmania/grpc,sreecha/grpc,ncteisen/grpc,geffzhang/grpc,matt-kwong/grpc,a11r/grpc,kskalski/grpc,muxi/grpc,philcleveland/grpc,ncteisen/grpc,vsco/grpc,makdharma/grpc,adelez/grpc,ipylypiv/grpc,pmarks-net/grpc,pmarks-net/grpc,infinit/grpc,kumaralokgithub/grpc,kriswuollett/grpc,dklempner/grpc,yugui/grpc,yang-g/grpc,PeterFaiman/ruby-grpc-minimal,grpc/grpc,deepaklukose/grpc,ctiller/grpc,infinit/grpc,apolcyn/grpc,fuchsia-mirror/third_party-grpc,sreecha/grpc,pszemus/grpc,MakMukhi/grpc,stanley-cheung/grpc,wcevans/grpc,Vizerai/grpc,fuchsia-mirror/third_party-grpc,pmarks-net/grpc,pmarks-net/grpc,deepaklukose/grpc,ipylypiv/grpc,makdharma/grpc,hstefan/grpc,murgatroid99/grpc,chrisdunelm/grpc,geffzhang/grpc,quizlet/grpc,deepaklukose/grpc,ejona86/grpc,vsco/grpc,thinkerou/grpc,apolcyn/grpc,thinkerou/grpc,dgquintas/grpc,dklempner/grpc,donnadionne/grpc,geffzhang/grpc,muxi/grpc,vjpai/grpc,Crevil/grpc,sreecha/grpc,infinit/grpc,dgquintas/grpc,grani/grpc,apolcyn/grpc,ipylypiv/grpc,simonkuang/grpc,mehrdada/grpc,PeterFaiman/ruby-grpc-minimal,royalharsh/grpc,pszemus/grpc,daniel-j-born/grpc,royalharsh/grpc,grpc/grpc,ncteisen/grpc,Crevil/grpc,infinit/grpc,carl-mastrangelo/grpc,yongni/grpc,chrisdunelm/grpc,hstefan/grpc,quizlet/grpc,kpayson64/grpc,donnadionne/grpc,sreecha/grpc,yugui/grpc,yang-g/grpc,PeterFaiman/ruby-grpc-minimal,jboeuf/grpc,ctiller/grpc,makdharma/grpc,kriswuollett/grpc,chrisdunelm/grpc,yang-g/grpc,royalharsh/grpc,pszemus/grpc,ejona86/grpc,kpayson64/grpc,baylabs/grpc,jtattermusch/grpc,pszemus/grpc,7anner/grpc,vjpai/grpc,carl-mastrangelo/grpc,jboeuf/grpc,ncteisen/grpc,kskalski/grpc,geffzhang/grpc,stanley-cheung/grpc,nicolasnoble/grpc,kpayson64/grpc,PeterFaiman/ruby-grpc-minimal,yongni/grpc,simonkuang/grpc,jtattermusch/grpc,LuminateWireless/grpc,kriswuollett/grpc,ncteisen/grpc,LuminateWireless/grpc,baylabs/grpc,fuchsia-mirror/third_party-grpc,murgatroid99/grpc,matt-kwong/grpc,rjshade/grpc,donnadionne/grpc,zhimingxie/grpc,kriswuollett/grpc,yongni/grpc,jtattermusch/grpc,stanley-cheung/grpc,sreecha/grpc,vsco/grpc,yugui/grpc,pszemus/grpc,firebase/grpc,grpc/grpc,nicolasnoble/grpc,hstefan/grpc,a11r/grpc,mehrdada/grpc,ncteisen/grpc,murgatroid99/grpc,jtattermusch/grpc,sreecha/grpc,vsco/grpc,baylabs/grpc,mehrdada/grpc,7anner/grpc,mehrdada/grpc,kskalski/grpc,sreecha/grpc,ctiller/grpc,chrisdunelm/grpc,wcevans/grpc,murgatroid99/grpc,simonkuang/grpc,thinkerou/grpc,hstefan/grpc,donnadionne/grpc,sreecha/grpc,pszemus/grpc,a11r/grpc,dgquintas/grpc,jboeuf/grpc,dklempner/grpc,PeterFaiman/ruby-grpc-minimal,thinkerou/grpc,simonkuang/grpc,firebase/grpc,pszemus/grpc,rjshade/grpc,murgatroid99/grpc,jtattermusch/grpc,chrisdunelm/grpc,vsco/grpc,apolcyn/grpc,muxi/grpc,firebase/grpc,adelez/grpc,hstefan/grpc,apolcyn/grpc,firebase/grpc,grani/grpc,pszemus/grpc,matt-kwong/grpc,royalharsh/grpc,Vizerai/grpc,muxi/grpc,adelez/grpc,chrisdunelm/grpc,jboeuf/grpc,stanley-cheung/grpc,kumaralokgithub/grpc,ctiller/grpc,jtattermusch/grpc,rjshade/grpc,Vizerai/grpc,ejona86/grpc,msmania/grpc,grpc/grpc,donnadionne/grpc,msmania/grpc,muxi/grpc,simonkuang/grpc,thinkerou/grpc,dgquintas/grpc,mehrdada/grpc,makdharma/grpc,nicolasnoble/grpc,dgquintas/grpc,vjpai/grpc,fuchsia-mirror/third_party-grpc,7anner/grpc,muxi/grpc,dgquintas/grpc,yugui/grpc,quizlet/grpc,baylabs/grpc,thinkerou/grpc,grani/grpc,msmania/grpc,ctiller/grpc,rjshade/grpc,a11r/grpc,grani/grpc,deepaklukose/grpc,kskalski/grpc,dgquintas/grpc,firebase/grpc,thinkerou/grpc,jtattermusch/grpc,geffzhang/grpc,geffzhang/grpc,fuchsia-mirror/third_party-grpc,mehrdada/grpc,dklempner/grpc,adelez/grpc,vjpai/grpc,yang-g/grpc,grpc/grpc,vjpai/grpc,kumaralokgithub/grpc,mehrdada/grpc,thinkerou/grpc,vjpai/grpc,wcevans/grpc,ncteisen/grpc,ctiller/grpc,stanley-cheung/grpc,7anner/grpc,msmania/grpc,matt-kwong/grpc,pszemus/grpc,pszemus/grpc,ncteisen/grpc,daniel-j-born/grpc,Crevil/grpc,thinkerou/grpc,firebase/grpc,kskalski/grpc,pmarks-net/grpc,msmania/grpc,mehrdada/grpc,rjshade/grpc,ejona86/grpc,thinkerou/grpc,quizlet/grpc,murgatroid99/grpc,msmania/grpc,nicolasnoble/grpc,Vizerai/grpc,zhimingxie/grpc,geffzhang/grpc,donnadionne/grpc,pmarks-net/grpc,vjpai/grpc,jboeuf/grpc,ejona86/grpc,yang-g/grpc,thinkerou/grpc,vjpai/grpc,kumaralokgithub/grpc,pmarks-net/grpc,firebase/grpc,infinit/grpc,zhimingxie/grpc,stanley-cheung/grpc,wcevans/grpc,MakMukhi/grpc,firebase/grpc,Crevil/grpc,baylabs/grpc,ipylypiv/grpc,muxi/grpc,ncteisen/grpc,LuminateWireless/grpc,carl-mastrangelo/grpc,jtattermusch/grpc,adelez/grpc,Vizerai/grpc,adelez/grpc,jboeuf/grpc,vsco/grpc,infinit/grpc,daniel-j-born/grpc,jtattermusch/grpc,royalharsh/grpc,ctiller/grpc,dgquintas/grpc,firebase/grpc,matt-kwong/grpc,vjpai/grpc,ejona86/grpc,deepaklukose/grpc,yugui/grpc,wcevans/grpc,chrisdunelm/grpc,nicolasnoble/grpc,chrisdunelm/grpc,kriswuollett/grpc,stanley-cheung/grpc,stanley-cheung/grpc,MakMukhi/grpc,vjpai/grpc,baylabs/grpc,royalharsh/grpc,Crevil/grpc,kpayson64/grpc,zhimingxie/grpc,wcevans/grpc,nicolasnoble/grpc,ejona86/grpc,sreecha/grpc,philcleveland/grpc,yugui/grpc,yongni/grpc,jtattermusch/grpc,makdharma/grpc,ipylypiv/grpc,Crevil/grpc,simonkuang/grpc,apolcyn/grpc,infinit/grpc,donnadionne/grpc,daniel-j-born/grpc,LuminateWireless/grpc,kskalski/grpc,jtattermusch/grpc,quizlet/grpc,philcleveland/grpc,mehrdada/grpc,kpayson64/grpc,ctiller/grpc,pszemus/grpc,sreecha/grpc,rjshade/grpc,kpayson64/grpc,grani/grpc,adelez/grpc,kumaralokgithub/grpc,donnadionne/grpc,carl-mastrangelo/grpc,jboeuf/grpc,nicolasnoble/grpc,daniel-j-born/grpc,PeterFaiman/ruby-grpc-minimal,nicolasnoble/grpc,grpc/grpc,ejona86/grpc,murgatroid99/grpc,mehrdada/grpc,philcleveland/grpc,fuchsia-mirror/third_party-grpc,makdharma/grpc,simonkuang/grpc,deepaklukose/grpc,jboeuf/grpc,a11r/grpc,carl-mastrangelo/grpc,matt-kwong/grpc,grani/grpc,pszemus/grpc,simonkuang/grpc,kskalski/grpc,carl-mastrangelo/grpc,makdharma/grpc,yugui/grpc,stanley-cheung/grpc,thinkerou/grpc,geffzhang/grpc,chrisdunelm/grpc,baylabs/grpc,zhimingxie/grpc,vsco/grpc,ncteisen/grpc,kpayson64/grpc,yongni/grpc,jtattermusch/grpc,kriswuollett/grpc,carl-mastrangelo/grpc,yongni/grpc,Crevil/grpc,hstefan/grpc,kpayson64/grpc,nicolasnoble/grpc,daniel-j-born/grpc,chrisdunelm/grpc,jboeuf/grpc,hstefan/grpc,jboeuf/grpc,a11r/grpc,Crevil/grpc,yang-g/grpc,rjshade/grpc,makdharma/grpc,apolcyn/grpc,grpc/grpc,philcleveland/grpc,ipylypiv/grpc,mehrdada/grpc,yang-g/grpc,firebase/grpc,Vizerai/grpc,ejona86/grpc,matt-kwong/grpc,ipylypiv/grpc,royalharsh/grpc,grpc/grpc,ctiller/grpc,murgatroid99/grpc,nicolasnoble/grpc,daniel-j-born/grpc,wcevans/grpc,vsco/grpc,pmarks-net/grpc,LuminateWireless/grpc,vjpai/grpc,Vizerai/grpc,kumaralokgithub/grpc,MakMukhi/grpc,simonkuang/grpc,rjshade/grpc,ncteisen/grpc,7anner/grpc,yongni/grpc,zhimingxie/grpc,Vizerai/grpc,muxi/grpc,adelez/grpc,stanley-cheung/grpc,msmania/grpc,msmania/grpc,kpayson64/grpc,ncteisen/grpc,yongni/grpc,fuchsia-mirror/third_party-grpc,jboeuf/grpc,rjshade/grpc,a11r/grpc,ctiller/grpc,philcleveland/grpc,quizlet/grpc,zhimingxie/grpc,PeterFaiman/ruby-grpc-minimal,firebase/grpc,carl-mastrangelo/grpc,ejona86/grpc,donnadionne/grpc,Vizerai/grpc,hstefan/grpc,sreecha/grpc,grani/grpc,donnadionne/grpc,ejona86/grpc,daniel-j-born/grpc,dklempner/grpc,muxi/grpc,a11r/grpc,philcleveland/grpc,royalharsh/grpc,7anner/grpc,LuminateWireless/grpc,ctiller/grpc,deepaklukose/grpc,PeterFaiman/ruby-grpc-minimal,dklempner/grpc,yang-g/grpc,dgquintas/grpc,ipylypiv/grpc,quizlet/grpc,grpc/grpc,pmarks-net/grpc,grpc/grpc,quizlet/grpc,Vizerai/grpc,adelez/grpc,zhimingxie/grpc,wcevans/grpc,kumaralokgithub/grpc,deepaklukose/grpc,dklempner/grpc,ejona86/grpc,geffzhang/grpc,PeterFaiman/ruby-grpc-minimal,murgatroid99/grpc,7anner/grpc,ipylypiv/grpc,kumaralokgithub/grpc,makdharma/grpc,nicolasnoble/grpc,kskalski/grpc,kriswuollett/grpc,matt-kwong/grpc,Vizerai/grpc,carl-mastrangelo/grpc,yugui/grpc,grani/grpc,firebase/grpc,yongni/grpc,dklempner/grpc,zhimingxie/grpc,stanley-cheung/grpc,kriswuollett/grpc,nicolasnoble/grpc,grpc/grpc,fuchsia-mirror/third_party-grpc,dgquintas/grpc,MakMukhi/grpc,kpayson64/grpc,a11r/grpc,kskalski/grpc,7anner/grpc,fuchsia-mirror/third_party-grpc,MakMukhi/grpc,baylabs/grpc,baylabs/grpc,donnadionne/grpc,vjpai/grpc,kumaralokgithub/grpc