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Currently the receiving of WM_MOUSEWHEEL messages lags when we open a
URL on a new window until we resize. The issue was introduced by
r384698. That CL change wait for MOUSE_EVENT to wait for SENDMESSAGE
in MessagePumpForUI::WaitForWork to optimize the GPU thread. But the
GPU thread does not use MessagePumpForUI anymore so it should be OK to
just remove that code.

The updated version of Chrome was tested to make sure that the hangs
that were occasionally seen when using chrome://tracing are still gone.

After this CL chaopeng@ need to monitor to make sure that the crash
rate not increase.

Bug: 713907, 596190
Change-Id: I7545a867de4851acd4e57c9b8c7a3dd05def1dd8
Reviewed-on: https://chromium-review.googlesource.com/809829
Commit-Queue: Jianpeng Chao <chaopeng@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523689}