Extend Blink mock devices to return frame rate on getSettings.

This CL adds a layout test that getSettings actually returns
frame rate. This required wiring up support for letting the
fake video source in blink_test_runner call its callbacks.

BUG=543997

Review-Url: https://codereview.chromium.org/2023203002
Cr-Commit-Position: refs/heads/master@{#397191}
2 files changed