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This allows using //base/task/post_task.h for posting tasks to a
BrowserThread by specifying a BrowserThread::ID as a task trait.

Also adds a content::NonNestable task trait to support non-nestable
tasks in base::PostTaskWithTraits.

In the future, we will add further traits to facilitate scheduling
tasks onto different SequenceManager queues on the UI thread, see:
https://docs.google.com/document/d/1z1BDq9vzcEpkhN9LSPF5XMnZ0kLJ8mWWkNAi4OI7cos/edit?usp=sharing

Bug: 867421, 863341, 878356
Change-Id: Id8b7bc2e374917ceb421c7f6139790e6f1457511
Reviewed-on: https://chromium-review.googlesource.com/1181364
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586728}