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* Start resizing upon resize request from client.
* Client can start drag only if
  1) mouse is pressed on the shell surface.
  2) gesture is on progress.

This CL includes a fix for potential issue when
the drag to resize/move was requested when the mouse/touch
drag wasn't in progress.

BUG: b/72918846
TEST: covered by unit test
Change-Id: Id32f9da7a2c718fdaef1cb0e4a4adb3b99cbd7f8
Reviewed-on: https://chromium-review.googlesource.com/938498
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539962}