MouseMove when the mouse is down to signal compositor priority

MouseMove when the mouse is down to signal compositor priority

When the user is moving the mouse with the mouse button down it's quite
likely they're scrolling the page, or otherwise doing something where
they would expect the animation to be smooth.  Use this as a signal that
we should enter compositor prioirty mode.  Note this does mean that with
the mouse down every MouseMove event will cause us to lock a mutex and
call Now() (which should be OK). A policy update task is only posted
every 100ms.

TBR=sievers@chromium.org for the FakeRenderScheduler
BUG=391005

Review URL: https://codereview.chromium.org/902783003

Cr-Commit-Position: refs/heads/master@{#315566}