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The Pepper plugin uses pp::Core::CallOnMainThread() to schedule tasks
on the main Pepper thread, while the non-Pepper plugin plans to use
base::SingleThreadTaskRunner() instead in the future.

This CL adds method ScheduleTaskOnMainThread() to PDFEngine::Client and
PaintManager::Client as a wrapper for pp::Core::CallOnMainThread() and
base::SingleThreadTaskRunner() calls, so that direct calls of these two
functions can be removed in the code shared by the Pepper and the
non-Pepper plugin.

Bug: 1108517
Change-Id: Ie1103adce2dbcf40ece40f318f7c3159f67e93ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2594508
Commit-Queue: Hui Yingst <nigi@chromium.org>
Reviewed-by: K. Moon <kmoon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838713}