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Previously, MediaSessionImpl uses the metadata and controls to determine
whether a page uses the MediaSession API. However there comes an issues
that we need to update the routed MediaSession every time when a service
is created/destroyed or its metadata/controls changed, which is a lot of
maintainance work.

This CL simplifies the MediaSessionService routing strategy. It only
checks whether a service is created for a given frame. The rationale is
that a MediaSessionService is created when and only when anything passes
through mojo, which is enough for indicating whether a frame uses
MediaSession API.

BUG=670319

Review-Url: https://codereview.chromium.org/2585243002
Cr-Commit-Position: refs/heads/master@{#439488}