Eliminate flakiness for inspector/device-orientation-success.html

R=pfeldman, vsevik

Review URL: https://codereview.chromium.org/52563003

git-svn-id: svn://svn.chromium.org/blink/trunk@160936 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: enable viewport in device emulation mode.

BUG=312580

Review URL: https://codereview.chromium.org/50043006

git-svn-id: svn://svn.chromium.org/blink/trunk@160937 bbb929c8-8fbe-4397-9dbb-9b2b20218538