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This patch introduces a ThreadPool task trait for marking tasks that
should run on the thread pool as opposed to a named browser thread such
as the main thread. Previously any tasks that did *not* indicate a named
thread would automatically run on the thread pool, which was convenient
but perhaps surprising, since these types of tasks can run concurrently
which may not be expected.

After a future patch, tasks will need to explicitly opt into either
thread pool or named thread execution; there is no automatic default.

Change-Id: I2d9cfcd8779b4af5a6ad7baece52bc6acbb7b737
Bug: 968047
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1632471
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664413}