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This is a reland of de5ec163

Reason for revert: rendering.desktop/frame_times regressions
Fix: SetWorkBatchSize(2) in gpu_main to keep previous behavior.

Original change's description:
> [base] Migrate the Mac/iOS MessagePump to DoSomeWork
>
> Recycling https://chromium-review.googlesource.com/c/chromium/src/+/1470938/13
> This CL also fixes DetachToBrowserTabDragControllerTest.PressEscapeWhileDetached
> to make CQ happy.
>
> Bug: 885371
> Change-Id: I0b54ade1fd3481a9248c89ec2993738b20fce002
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906337
> Reviewed-by: Allen Bauer <kylixrd@chromium.org>
> Reviewed-by: Mark Mentovai <mark@chromium.org>
> Reviewed-by: Robert Sesek <rsesek@chromium.org>
> Reviewed-by: Gabriel Charette <gab@chromium.org>
> Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#722094}

Bug: 885371, 1032188
Change-Id: Id1d6916a83e1c72cacc554c100d60fd0c78c6b7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992161
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732010}