Update window.devicePixelRatio on browser zoom

Update window.devicePixelRatio on full page zoom so that it
accurately portraitists the ratio between css and device pixels.

This matches the behavior of Firefox and the IE 11 preview.

BUG=177836
R=eseidel@chromium.org, dglazkov@chromium.org
TEST=fast/dom/Window/device-pixel-ratio-on-zoom.html

Review URL: https://chromiumcodereview.appspot.com/23000005

git-svn-id: svn://svn.chromium.org/blink/trunk@156040 bbb929c8-8fbe-4397-9dbb-9b2b20218538