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This CL lands the Mojo service that we intend to use for Background
Fetch. Everything but the fetch() event has been hooked up from the
renderer side, and will be received by a strongly binded service
implementation in the browser process.

The fetch() function will be implemented separately, as it will require
some plumbing to support the appropriate types for requests.

BUG=692534, 692535

Review-Url: https://codereview.chromium.org/2745243002
Cr-Commit-Position: refs/heads/master@{#457490}