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This leak was introduced by https://crrev.com/c/1462801/ where
there was a confusion between MessageLoop and MessageLoopBase
(surprisingly the former owns the latter).

This patch fixes the leak and generally clarifies ownership.
We still need to change base::Thread::Options::task_environment to be a
std::unique_ptr but that's a larger refactor.

Change-Id: Ic5380df2fb9dca5cb0818ce8e8b90e05192c6e9d
BUG: 933925, 934088, 933014, 932857
Reviewed-on: https://chromium-review.googlesource.com/c/1480462
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634462}