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The new monitor's real DPI would override the forced scale factor, causing
DCHECK failures.  Instead, use the forced scale factor if it exists.

Bug: none
Test: Start a debug build of chrome on a two-monitor system with --force-device-scale-factor=1.5 (or some other value that doesn't match your monitors), drag the window from one monitor to the other.  This shouldn't cause a DCHECK failure.
Change-Id: I5ef336add05840f7ec2d7277b54855ef05e17d28
Reviewed-on: https://chromium-review.googlesource.com/1137481Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575513}