gpu: Bind dummy GL API when no context is current
Also make platform behavior consistent in always releasing
any previously current context when MakeCurrent() fails.
This catches GL call sites with no context current.
It also avoids problems with GL implementations potentially not liking
this (and crashing) or even us ending up calling into the wrong context
(for example accidentally deleting a resource in the wrong context).
BUG=355275
R=piman@chromium.org
Review URL: https://codereview.chromium.org/
221433004
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@261153 0039d316-1c4b-4281-b951-d872f2087c98