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This adds the mimeHandler extension API, which is a simplified version
of the streamsPrivateApi. Unlike the streamsPrivateApi, mimeHandler only
allows access to streams from RenderFrames within MimeHandlerViewGuests,
and in particular only allows access to the stream being handled by that
MimeHandlerViewGuest instance. This removes the need for an event to be
dispatched to the background page and a user-exposed view_id.

BUG=439867

Review URL: https://codereview.chromium.org/797183005

Cr-Commit-Position: refs/heads/master@{#312728}