[DevTools] Mimic pageScaleFactor settings from Android when emulating viewport.

BUG=384563,377501

Review URL: https://codereview.chromium.org/350853003

git-svn-id: svn://svn.chromium.org/blink/trunk@176978 bbb929c8-8fbe-4397-9dbb-9b2b20218538