Add Settings::deviceScaleAdjustment.

This will be used to plumb the 1.05-1.3x fudge factor separately from the font
scale factor.  It is applied to the autosizer's cluster multiplier only when
the page does not set an explicit meta viewport or @viewport.

BUG=252828

Review URL: https://codereview.chromium.org/49913005

git-svn-id: svn://svn.chromium.org/blink/trunk@160957 bbb929c8-8fbe-4397-9dbb-9b2b20218538