[DevTools] Use device metrics emulation implemented in content.

      Device emulation is now split: while metrics emulation is simply forwarded
to embedder, a number of internal blink settings are set to emulate device
behavior. These settings include overlay scrollbars, viewport support, text
autosizing and page scaling.

To support fit-to-window mode, WebView now sets a scaling transform
to the root graphics layer. Also, device scale factor is overriden to produce
sharp image even when emulating.

This also removes old device metrics emulation support through blink code.

See also Chromium-side patch: https://codereview.chromium.org/23364004/

BUG=288959
R=aelias@chromium.org, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/23187005

git-svn-id: svn://svn.chromium.org/blink/trunk@159110 bbb929c8-8fbe-4397-9dbb-9b2b20218538