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Jank analysis uncovered that Windows uses native ::PeekMessage calls as
an opportunity to yield to other threads according to some heuristics
(e.g. presumably when there's no input but perhaps a single WM_USER
message posted later than another thread was readied). MessagePumpForUI
doesn't intend to give this opportunity to the kernel when invoking
::PeekMessage however as it runs most tasks out-of-band. Hence,
PM_NOYIELD should be used to tell ::PeekMessage it's not the only source
of work for this thread.

R=fdoray@chromium.org

Bug: 1075960
Change-Id: Ic8808cab8c15aa690f7dfd8405741323285c107b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2169728
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763349}