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When the mouse cursor exits from the browser window,
'CompoundEventFilter' doesn't update the cursor and it doesn't
know whether ET_MOUSE_EXITED happens when the window is destroying
or the mouse exits from the window. WindowTreeHostPlatform keeps
the previous cursor and WaylandWindow, one of PlatformWindows,
also keeps it.

This patch resets the cursor from WindowTreeHostPlatform on
ET_MOUSE_EXITED to update it when it reenters the window.

Test: aura_unittests
Bug: 879448
Change-Id: I52919aaadef4a0934eb6ad88c1f4d4ba75f802b0
Reviewed-on: https://chromium-review.googlesource.com/1198714
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#589812}