Windows: Fixed warnings in `windows/os.hpp`.
Windows: Fixed warnings in `windows/os.hpp`.

Captured the result of `Process32First` via its return type `BOOL`
rather than implicitly casting to `bool`.

Review: https://reviews.apache.org/r/52063/

gsantovena/mesos,verizonlabs/mesos,asamerh4/mesos,yuquanshan/customizedMesos,jpeach/mesos,Gilbert88/mesos,chhsia0/mesos,gsantovena/mesos,asamerh4/mesos,yuquanshan/customizedMesos,yuquanshan/customizedMesos,craimbert/mesos,asamerh4/mesos,andschwa/mesos,neilconway/mesos,neilconway/mesos,reneploetz/mesos,gsantovena/mesos,zmalik/mesos,kaysoky/mesos,verizonlabs/mesos,andschwa/mesos,jpeach/mesos,Gilbert88/mesos,kaysoky/mesos,zmalik/mesos,zmalik/mesos,Aman-Jain-14/customizedMesos,abudnik/mesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,craimbert/mesos,jpeach/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,andschwa/mesos,andschwa/mesos,reneploetz/mesos,dforsyth/mesos,chhsia0/mesos,kaysoky/mesos,dforsyth/mesos,dforsyth/mesos,yuquanshan/customizedMesos,reneploetz/mesos,reneploetz/mesos,andschwa/mesos,abudnik/mesos,kaysoky/mesos,gsantovena/mesos,dforsyth/mesos,zmalik/mesos,reneploetz/mesos,reneploetz/mesos,reneploetz/mesos,neilconway/mesos,verizonlabs/mesos,abudnik/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,jpeach/mesos,Gilbert88/mesos,neilconway/mesos,Gilbert88/mesos,gsantovena/mesos,chhsia0/mesos,kaysoky/mesos,craimbert/mesos,asamerh4/mesos,jpeach/mesos,dforsyth/mesos,craimbert/mesos,craimbert/mesos,chhsia0/mesos,neilconway/mesos,Gilbert88/mesos,andschwa/mesos,yuquanshan/customizedMesos,asamerh4/mesos,neilconway/mesos,verizonlabs/mesos,kaysoky/mesos,chhsia0/mesos,asamerh4/mesos,Aman-Jain-14/customizedMesos,abudnik/mesos,jpeach/mesos,Gilbert88/mesos,abudnik/mesos,dforsyth/mesos,yuquanshan/customizedMesos,zmalik/mesos,verizonlabs/mesos,zmalik/mesos,dforsyth/mesos,zmalik/mesos,andschwa/mesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos,craimbert/mesos,jpeach/mesos,yuquanshan/customizedMesos,chhsia0/mesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos,chhsia0/mesos,neilconway/mesos,abudnik/mesos,abudnik/mesos,gsantovena/mesos,asamerh4/mesos,craimbert/mesos