Implement the Platform::sharedOffscreenGraphicsContext3D method.

This method is being added to the Platform API for WebKit, and
when it is added, DumpRenderTree will use it for its shared main thread context.

R=jamesr,piman
BUG=177768
TBR=darin

Review URL: https://codereview.chromium.org/12217099

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@185357 0039d316-1c4b-4281-b951-d872f2087c98