Update C++ code generation to work with Bazel 0.29 .
Update C++ code generation to work with Bazel 0.29 .

The above Bazel version changes proto compilation slightly: some proto
files are put into a `_virtual_imports` directory and thus
`_get_include_directory` needs to be updated accordingly.

Ideally, it would use instead the `ProtoInfo` provider to tease out the
proto import directories, but that's a bit more intrusive change.

grpc/grpc,nicolasnoble/grpc,pszemus/grpc,ctiller/grpc,donnadionne/grpc,muxi/grpc,firebase/grpc,ejona86/grpc,grpc/grpc,ejona86/grpc,firebase/grpc,jboeuf/grpc,ejona86/grpc,stanley-cheung/grpc,ejona86/grpc,stanley-cheung/grpc,ejona86/grpc,grpc/grpc,muxi/grpc,ctiller/grpc,stanley-cheung/grpc,nicolasnoble/grpc,pszemus/grpc,ctiller/grpc,ejona86/grpc,stanley-cheung/grpc,grpc/grpc,jtattermusch/grpc,jboeuf/grpc,jboeuf/grpc,pszemus/grpc,grpc/grpc,vjpai/grpc,jtattermusch/grpc,ejona86/grpc,pszemus/grpc,jboeuf/grpc,vjpai/grpc,firebase/grpc,ctiller/grpc,jtattermusch/grpc,nicolasnoble/grpc,firebase/grpc,stanley-cheung/grpc,donnadionne/grpc,jtattermusch/grpc,vjpai/grpc,firebase/grpc,vjpai/grpc,jtattermusch/grpc,firebase/grpc,vjpai/grpc,donnadionne/grpc,ctiller/grpc,firebase/grpc,jboeuf/grpc,pszemus/grpc,stanley-cheung/grpc,grpc/grpc,jtattermusch/grpc,pszemus/grpc,pszemus/grpc,nicolasnoble/grpc,stanley-cheung/grpc,muxi/grpc,jtattermusch/grpc,ctiller/grpc,grpc/grpc,vjpai/grpc,ctiller/grpc,ejona86/grpc,muxi/grpc,nicolasnoble/grpc,ctiller/grpc,grpc/grpc,nicolasnoble/grpc,ejona86/grpc,nicolasnoble/grpc,jboeuf/grpc,vjpai/grpc,muxi/grpc,muxi/grpc,nicolasnoble/grpc,muxi/grpc,firebase/grpc,vjpai/grpc,donnadionne/grpc,nicolasnoble/grpc,ctiller/grpc,donnadionne/grpc,firebase/grpc,jtattermusch/grpc,vjpai/grpc,pszemus/grpc,donnadionne/grpc,nicolasnoble/grpc,jboeuf/grpc,jboeuf/grpc,firebase/grpc,jboeuf/grpc,pszemus/grpc,stanley-cheung/grpc,pszemus/grpc,muxi/grpc,pszemus/grpc,grpc/grpc,jtattermusch/grpc,stanley-cheung/grpc,jboeuf/grpc,jtattermusch/grpc,firebase/grpc,grpc/grpc,ejona86/grpc,nicolasnoble/grpc,firebase/grpc,stanley-cheung/grpc,donnadionne/grpc,pszemus/grpc,nicolasnoble/grpc,grpc/grpc,vjpai/grpc,muxi/grpc,ctiller/grpc,ctiller/grpc,muxi/grpc,jtattermusch/grpc,muxi/grpc,jtattermusch/grpc,jboeuf/grpc,vjpai/grpc,ctiller/grpc,ejona86/grpc,stanley-cheung/grpc,stanley-cheung/grpc,donnadionne/grpc,donnadionne/grpc,grpc/grpc,muxi/grpc,ejona86/grpc,donnadionne/grpc,donnadionne/grpc,vjpai/grpc,donnadionne/grpc,jboeuf/grpc