[DevTools] Device emulation sanity test.

Turned out that layout tests didn't catch compositor checks.
DevTools sanity test should help here.

BUG=370035

Review URL: https://codereview.chromium.org/270783004

git-svn-id: svn://svn.chromium.org/blink/trunk@173870 bbb929c8-8fbe-4397-9dbb-9b2b20218538