[DevTools] Add offset and scale to WebDeviceEmulationParams.

This will be used to set offset/scale from DevTools protocol
instead of caluculating them in embedder.

BUG=327641
NOTRY=true

Review URL: https://codereview.chromium.org/342953003

git-svn-id: svn://svn.chromium.org/blink/trunk@176506 bbb929c8-8fbe-4397-9dbb-9b2b20218538