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Attempt to init the display device scale factor in WindowTreeHost's ctor.
WindowTreeHost subclasses/users may use the cached scale before InitHost.
See http://crbug.com/899084 and https://crrev.com/c/1311080 (PS7)

This may incorrectly use the primary display's scale for new windows on extended displays.
Still, a best effort attempt to initialize the cached value here may be worthwhile.
It's unclear if this solves any practical defects similar to crbug.com/899084.

Bug: None
Test: Automated
Change-Id: Ia776d870d8a51c5ac381b2a023c7a2a579dca97a
Reviewed-on: https://chromium-review.googlesource.com/c/1318693Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605770}