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Previously, Display would read the device scale factor from the command
line in its ctor, but then screen would clobber it.  This meant that
the force device scale factor command line flag was silently ignored.

R=oshima@chromium.org,ccameron@chromium.org

Review URL: https://codereview.chromium.org/1111543003

Cr-Commit-Position: refs/heads/master@{#327211}