[merge m57] Prefer returning a SingleThreadTaskRunner in SequencedTaskRunnerHandle::Get().

[merge m57] Prefer returning a SingleThreadTaskRunner in SequencedTaskRunnerHandle::Get().

Currently:
- When a thread has a SingleThreadTaskRunner but not
  SequencedTaskRunner, SequencedTaskRunnerHandle::Get() and
  ThreadTaskRunnerHandle::Get() are equivalent.
- ThreadTaskRunnerHandle::Get() can only be called from a thread that
  has a SingleThreadTaskRunner.
- Having both a SequencedTaskRunner and a SingleThreadTaskRunner on the
  same thread is prevented by a DCHECK.
That means that replacing ThreadTaskRunnerHandle::Get() with
SequencedTaskRunnerHandle::Get() shouldn't change anything.

Unfortunately, RLZ sets a SingleThreadTaskRunner on a thread that
already has a SequencedTaskRunner.

To make the replacement of ThreadTaskRunnerHandle::Get() with
SequencedTaskRunnerHandle::Get() a true no-op under the assumption
that a SingleThreadTaskRunner can be registered on a thread that
already has a SequencedTaskRunner, this CL gives priority to the
SingleThreadTaskRunner in SequencedTaskRunnerHandle::Get().

This change will be reverted once RLZ stops setting a
SingleThreadTaskRunner on a thread that already has a
SequencedTaskRunner.

BUG=693711, 618530
Review-Url: https://codereview.chromium.org/2714813005
Cr-Commit-Position: refs/heads/master@{#452980}
(cherry picked from commit 533c31e78be276dcaa843dc66efd4fee697d9573)

Review-Url: https://codereview.chromium.org/2725873006 .
Cr-Commit-Position: refs/branch-heads/2987@{#748}
Cr-Branched-From: ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed: https://chromium.googlesource.com/chromium/src/+/06bad34f6516d13a5b39beea0e575431f0a803a2

[fdoray, 2017-03-03 03:57:37]

Description was changed from

==========
[merge m57] Prefer returning a SingleThreadTaskRunner in
SequencedTaskRunnerHandle::Get().

Currently:
- When a thread has a SingleThreadTaskRunner but not
  SequencedTaskRunner, SequencedTaskRunnerHandle::Get() and
  ThreadTaskRunnerHandle::Get() are equivalent.
- ThreadTaskRunnerHandle::Get() can only be called from a thread that
  has a SingleThreadTaskRunner.
- Having both a SequencedTaskRunner and a SingleThreadTaskRunner on the
  same thread is prevented by a DCHECK.
That means that replacing ThreadTaskRunnerHandle::Get() with
SequencedTaskRunnerHandle::Get() shouldn't change anything.

Unfortunately, RLZ sets a SingleThreadTaskRunner on a thread that
already has a SequencedTaskRunner.

To make the replacement of ThreadTaskRunnerHandle::Get() with
SequencedTaskRunnerHandle::Get() a true no-op under the assumption
that a SingleThreadTaskRunner can be registered on a thread that
already has a SequencedTaskRunner, this CL gives priority to the
SingleThreadTaskRunner in SequencedTaskRunnerHandle::Get().

This change will be reverted once RLZ stops setting a
SingleThreadTaskRunner on a thread that already has a
SequencedTaskRunner.

BUG=693711, 618530

Review-Url: https://codereview.chromium.org/2714813005
Cr-Commit-Position: refs/heads/master@{#452980}
(cherry picked from commit 533c31e78be276dcaa843dc66efd4fee697d9573)
==========

to

==========
[merge m57] Prefer returning a SingleThreadTaskRunner in
SequencedTaskRunnerHandle::Get().

Currently:
- When a thread has a SingleThreadTaskRunner but not
  SequencedTaskRunner, SequencedTaskRunnerHandle::Get() and
  ThreadTaskRunnerHandle::Get() are equivalent.
- ThreadTaskRunnerHandle::Get() can only be called from a thread that
  has a SingleThreadTaskRunner.
- Having both a SequencedTaskRunner and a SingleThreadTaskRunner on the
  same thread is prevented by a DCHECK.
That means that replacing ThreadTaskRunnerHandle::Get() with
SequencedTaskRunnerHandle::Get() shouldn't change anything.

Unfortunately, RLZ sets a SingleThreadTaskRunner on a thread that
already has a SequencedTaskRunner.

To make the replacement of ThreadTaskRunnerHandle::Get() with
SequencedTaskRunnerHandle::Get() a true no-op under the assumption
that a SingleThreadTaskRunner can be registered on a thread that
already has a SequencedTaskRunner, this CL gives priority to the
SingleThreadTaskRunner in SequencedTaskRunnerHandle::Get().

This change will be reverted once RLZ stops setting a
SingleThreadTaskRunner on a thread that already has a
SequencedTaskRunner.

BUG=693711, 618530

Review-Url: https://codereview.chromium.org/2714813005
Cr-Commit-Position: refs/heads/master@{#452980}
(cherry picked from commit 533c31e78be276dcaa843dc66efd4fee697d9573)

Review-Url: https://codereview.chromium.org/2725873006 .
Cr-Commit-Position: refs/branch-heads/2987@{#748}
Cr-Branched-From:
ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed:
https://chromium.googlesource.com/chromium/src/+/06bad34f6516d13a5b39beea0e57...
==========

[fdoray, 2017-03-03 03:57:39]

Committed patchset #1 (id:1) manually as
06bad34f6516d13a5b39beea0e575431f0a803a2.