aura: Attach lost context callback to the shared main thread context

r225643 broke lost context recovery on Aura with the thread (i.e. Chrome OS).
The lost context callback needs to be attached to the shared main thread
context, since that's what creates the resources - the offscreen one may not
even exist / be used.

BUG=none

Review URL: https://codereview.chromium.org/25367003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226610 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226610
Remove ui_test_utils::RegisterAndWait and use WindowedNotificationObserver et al. instead.

RegisterAndWait can lead to flakiness, as it relies on someone quitting the messageloop, which could happen earlier than expected.

BUG=284385

Review URL: https://codereview.chromium.org/23876024

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226613 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226613