Add ".../third_party/protobuf/src" to the include_dirs list

Fixes "fatal error: 'google/protobuf/stubs/common.h' file not found".

brightray/brightray.gyp

@@ -25,6 +25,7 @@
         '<(libchromiumcontent_src_dir)/third_party/mojo/src',
         '<(libchromiumcontent_src_dir)/third_party/WebKit',
         '<(libchromiumcontent_src_dir)/third_party/khronos',
+        '<(libchromiumcontent_src_dir)/third_party/protobuf/src',
         '<(libchromiumcontent_dir)/gen',
       ],
       'direct_dependent_settings': {