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This is used to satisfy Env::Schedule calls. The implementation lazily
spins a new PlatformThread and maintains a queue of tasks for it to
run. LevelDB uses this to schedule work which may do file I/O. For the
sandboxed Storage Service, this means doing sync Mojo IPCs.

Mojo IPC in general requires some basic task scheduling state to be set
up on the calling thread though, so the Storage Service will DCHECK
when it tries to do compaction within a sandbox.

Fortunately this thread doesn't seem to be necessary anymore, and it can
be replaced with a much simpler dispatch to the ThreadPool. Env contract
does not require scheduled tasks to be serialized, so they can be thrown
at the ThreadPool without a sequenced task runner.

A test is added to the sandboxed Storage Service in order to provide
some (albeit roundabout) coverage of this code path.

Incidentally the test also revealed a bug in FilesystemProxy which
caused it to completely ignore its given IPC task runner, which can
cause deadlocks in certain situations. This is also fixed and
effectively covered by the new test.

Bug: 1052045
Change-Id: Ic299a4050240c1b52d3669964ff6b65f74de8179
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2341415Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#796599}