[DevTools] Device emulation sanity test.
[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: bf5cd6ccde378db821296732a091cfbcf5285fbd@173870 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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