Change devicePixelRatio to include full-page zoom

Change window.devicePixelRatio to take full-page zoom (but not pinch
zoom) into account. Matches behavior of FF and IE11.

Approved to ship as part of blink launch process:
https://groups.google.com/a/chromium.org/d/topic/blink-dev/RyUSi3zdumQ/discussion

BUG=279430

Review URL: https://chromiumcodereview.appspot.com/23441002

git-svn-id: svn://svn.chromium.org/blink/trunk@156736 bbb929c8-8fbe-4397-9dbb-9b2b20218538