Make inspecting incognito extension background pages work again.

BUG=297856

Review URL: https://codereview.chromium.org/24460002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243699 0039d316-1c4b-4281-b951-d872f2087c98
Add GrabWindowSnapshotAsync aura tests

This adds a version of GrabWindowSnapshotAsync that behaves identically
to GrabWindowSnapshot except with a callback.  The snapshot aura unit
tests are updated to use this new async function instead of the sync
one.

In order to make the snapshot tests work properly, they also now have to
use a real gl context so that real drawing and readback can occur
through the compositor.

BUG=331410, 332167

Review URL: https://codereview.chromium.org/119753007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243700 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243700