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The |service_| weak pointer in PaintPreviewCompositorClientImpl appears
to be getting invalidated on a wrong thread in some in the wild crash
reports. This CL moves the deleter of PaintPreviewCompositorClient and
PaintPreviewCompositorService to always run on the default task runner
this should ensure the WeakPtr is invalidated on the right thread.

Bug: 1120313
Change-Id: I35f766c58c72537f96022131029008d2a0f6c603
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2368462Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800604}