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The problem this fixes is, depending on the exact timing, a context would
receive a lost context callback (task 1), and would then post a task to recreate
the context (task 2). In parallel a task would be posted to signal the
GpuChannelHost that the channel is lost (task 3).

Because task 3 is posted from the IO thread, whereas task 2 is posted from
another thread (but they both are posted to the "main" thread), the race in
posting results in a race in execution, and task 2 may try to recreate a context
using a channel that doesn't know yet that it's non-functional, resulting in
failing to send the message and failing to create the context.

This patch changes the post order - task 3 is posted before task 1 - ensuring
task 2 is posted after task 3 (since task 2 is posted after task 1 executes).

BUG=129067


Review URL: https://chromiumcodereview.appspot.com/10837104

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@149976 0039d316-1c4b-4281-b951-d872f2087c98