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This can be used from the renderer process or the browser
process. Requesting the manifest can be done via one call, a
callback has to be passed and will run with the manifest
passed as parameter. A failure will return the empty
manifest.

Some more logic could be added like caching the manifest in
the browser process or keeping track of the manifest dirty
state in the browser process but those things can be added
transparently later.

BUG=366145

Review URL: https://codereview.chromium.org/537053002

Cr-Commit-Position: refs/heads/master@{#295085}