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For code simplification, this CL merges mojom::PrerenderHandle into
mojom::PrerenerProcessor. These interfaces were very similar: These were
materialized per <link rel=prerender> element, and used for sending
messages from a renderer process to the browser process.

To merge them, this CL...

- assigns unique identifier per LinkPreload instance. This is used for
  canceling or abandoning prerendering from PrerenderProcessorImpl.
- removes PrerenderHandleProxy that implemented mojom::PrerenderHandle.

Bonus: This cleanup will make it easier to reuse the mojom interfaces
for the prerender V2 implementation we are now planning (see the issue).

Change-Id: If731c50408c4e125b3e71a70bef9bbf680eb073f
Bug: 1126305
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2413830Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810968}