Support framebufferTexture2DMultisampleEXT in webgraphicscontext3d

This will allow us to use the extension from blink.

BUG=322706

Review URL: https://codereview.chromium.org/105273006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240575 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240575