Implementation of device metrics emulation in render view.

This is used by DevTools to emulate various devices on desktop browser.
Emulation includes changing view size, view/window screen positions and
device scale factor. Also, the whole view is scaled down if required to
fit the browser window.

To achieve this, render widget creates an emulator, which handles metrics
override by intercepting size/position-related messages.
Scaling down is done by setting transform on the root composited layer
on the blink side.

This emulation is transparent to browser side.

Depends on Blink-side patch: https://codereview.chromium.org/23187005/

BUG=288959

Review URL: https://codereview.chromium.org/23364004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226663 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226663