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Clairfy the correct thread behaviors for PaintPreviewClient and
PaintPreviewBaseService.

Largely just a documentation change.

Change-Id: Ia1f9cadc8420a80610b3fd8ddb17d77bae3b12d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2090198Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747780}