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This reverts commit a1037c1c.

Reason for revert: https://crbug.com/657959#c47

Original change's description:
> Change FrameSinkManager task runner.
> 
> On Mac there is a special task runner that runs when the main task
> runner is blocked on resize. The main task runner blocks until a
> CompositorFrame arrives, and we need HostFrameSinkManager and
> FrameSinkManagerImpl to continue processing messages during this time.
> 
> Bug: 657959
> Change-Id: I3d8bb8bb22b5a4d36d1fe83e1144d11bbe1fd59d
> Reviewed-on: https://chromium-review.googlesource.com/563860
> Commit-Queue: kylechar <kylechar@chromium.org>
> Reviewed-by: Saman Sami <samans@chromium.org>
> Reviewed-by: Fady Samuel <fsamuel@chromium.org>
> Reviewed-by: Antoine Labour <piman@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#486070}

TBR=fsamuel@chromium.org,kylechar@chromium.org,piman@chromium.org,samans@chromium.org

Change-Id: I540b79a8fc55ec260268bf437903e05020dc398f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 657959
Reviewed-on: https://chromium-review.googlesource.com/568879Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486088}