Fixed build problems on Mac OS X when using GLContext from Chromium's

WebKit code. Added glIsFramebufferEXT and glIsRenderbufferEXT as
preferred aliases.

BUG=none
TEST=WebGL, try bots

Review URL: http://codereview.chromium.org/3146002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@55584 0039d316-1c4b-4281-b951-d872f2087c98