Fix --force-device-scale-factor on Mac

Fix --force-device-scale-factor on Mac

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

Committed: https://crrev.com/a9d8ec8211082f6063f1f9188d7585fe05362f58
Cr-Commit-Position: refs/heads/master@{#327211}

[ccameron, 2015-04-28 00:18:28]

lgtm -- thanks!

[enne (OOO), 2015-04-28 00:32:18]

The CQ bit was checked by enne@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-04-28 00:33:23]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1111543003/1

[oshima, 2015-04-28 00:46:32]

lgtm

[enne (OOO), 2015-04-28 01:16:14]

The CQ bit was unchecked by enne@chromium.org

[enne (OOO), 2015-04-28 01:16:16]

The CQ bit was checked by enne@chromium.org

[commit-bot: I haz the power, 2015-04-28 01:16:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1111543003/1

[commit-bot: I haz the power, 2015-04-28 01:19:24]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-28 01:21:16]

Patchset 1 (id:??) landed as
https://crrev.com/a9d8ec8211082f6063f1f9188d7585fe05362f58
Cr-Commit-Position: refs/heads/master@{#327211}