Cherry pick r512 from upstream protobuf

Fixes compilation on VS2013.

TBR=cpu@chromium.org
BUG=288498

Review URL: https://codereview.chromium.org/27017004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228723 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228723