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
3 files changed