DevTools: Screencast view scaling

We request the image of max size avaliable but resize canvas to match the size of actual image returned from device.

Depends on https://codereview.chromium.org/351633002/

BUG=387557

Review URL: https://codereview.chromium.org/347143002

git-svn-id: svn://svn.chromium.org/blink/trunk@177113 bbb929c8-8fbe-4397-9dbb-9b2b20218538