Avoid repeated command-line checks for forced DPI scale

Every time |Screen.GetPrimaryDisplay()| is accessed on Android, which can be
multiple times a frame, we're querying the command-line for
kForceDeviceScaleFactor.  Instead, cache the result of the query, saving ~8us
(50% of the cost) per GetPrimaryDisplay() call on a typical Android device.

BUG=341613

Review URL: https://codereview.chromium.org/186733009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255524 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255524