Add offscreen context creation attributes to GGL.

View contexts and more extensive color format picking not handled yet.

BUG=39849
TEST=WebGL conformance tests (context*.html)

Review URL: http://codereview.chromium.org/3302019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@61220 0039d316-1c4b-4281-b951-d872f2087c98
25 files changed