Unify gyp rules for running protoc.

- Add a protoc.gypi that can be gyp-included into any gyp file that
  wants to build .proto files.
- Convert two remoting gyp files to use this new protoc.gypi.

(Also fixes a bug in one of those remoting gyp files; a mistaken path
was causing it to always rebuild under Xcode.)

Review URL: http://codereview.chromium.org/7621073

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@97366 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed