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This is a preparation for removing webkit-prefixed prerendering events
(see https://crbug.com/839030). mojom::PrerepderProcessorClient is used
for dispatching the events and will be removed along with the events, so
it won't be available for monitoring the connection.

After this CL, mojom::PrerenderProcessor is used for monitoring the
connection instead of mojom::PrerenderProcessorClient so that follow-up
CLs can remove it. The renderer-side endpoints of
mojom::PrerenderProcessor and mojom::PrerenderProcessorClient have the
same lifetime, so the timing of disconnection is not changed
before/after this CL.

Another notable point in this CL is to change the ownership of
PrerenderProcessorImpl in the browser process. Before this CL,
PrerenderProcessorImpl is owned by mojo::MakeSelfOwnedReceiver. After
this CL, the processor destroys itself on the disconnect handler. This
change is necessary for monitoring the disconnection because
mojo::MakeSelfOwnedReceiver doesn't provide any hook point to register
the discconnect handler.

Bug: 839030
Change-Id: I36a094466690d352e1c149792a345ebdaf8a32d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522725Reviewed-by: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827590}