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Views client is supposed to use Widget::SetCursor so that
it can tell the correct cursor when asked via GetCursor on Widget.

Exo pointer should follow the same pattern and Arc notification
should simply use the cursor on the widget.

TBR=yoshiki@chromium.org
BUG=798056
TEST=manual. Cursor does not flicker on ARC notifications

Change-Id: I09e93526181c9707786ff1a9b0eb4cb682e6026b
Reviewed-on: https://chromium-review.googlesource.com/903223Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534601}