GitLab

The callbacks passed to PaintPreviewClient were getting run on an
arbitrary thread via PostTask. This is a problem as they should be run
on the calling thread such that WeakPtrs bound to them are valid.

Fix
- Make the callbacks run on the same thread that called capture.
- Explicitly run CapturePaintPreviewInternal on the UI thread.

Follow-up
- Make a dedicated SequencedTaskRunner for executing captures.

Bug: 1058079
Change-Id: I1dfa969de613112b4bd1650051e394094dffda5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083567
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746517}