Add font scale factor override to the inspector.

The font scale factor provided by the Android system is important for
accurate text sizing. This patch adds this setting back to the
viewport overrides section of the inspector, and correctly computes it
on a per-device setting.

This improves autosizing behavior so that sites are almost exactly as
they appear on the real device.

BUG=302005

Review URL: https://codereview.chromium.org/50343002

git-svn-id: svn://svn.chromium.org/blink/trunk@161027 bbb929c8-8fbe-4397-9dbb-9b2b20218538