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This CL makes MediaSession non user-constructible, and make it as
an attribute of window.navigator, thus MediaSession is one object
per frame. This CL also adds back the removed browser tests as
layout tests using a mock blink::mojom::MediaSessionService in js.
This CL also made MediaSession enabled in tests so no need for
virtual tests anymore.

Spec URL: https://wicg.github.io/mediasession/

BUG=643190,646490,653894

Review-Url: https://codereview.chromium.org/2401013002
Cr-Commit-Position: refs/heads/master@{#424702}