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Three changes:

1- Avoid returning a raw pointer to chrome::mojom::PrerenderCanceler
   from GetPrerenderCanceller helper functions in favor returning a
   mojo::PendingRemote<chrome::mojom::PrerenderCanceler>. This avoids
   the need to schedule a DeleteSoon (in the renderer case) to cleanup
   the memory associated with that object later.

2- By returning a mojo::PendingRemote<..> instead of a raw pointer to
   the interface, it is no longer necessary to manually PostTask before
   accessing that interface. A mojo::PendingRemote<..> can instead be
   bound to a mojo::Remote<..> that implicitly schedules function calls
   on the right thread. The PostTask is implicit.

3- Make PrerenderContents support having multiple bound cancelers. This
   makes the above possible. In the renderer, the canceler is acquired
   immediately instead of lazily, and that is extra cost (an extra IPC)
   in the case where a canceler is not needed. This only happens for a
   page being prerendered, and so that overhead should be relatively
   small and seems worth the benefit of simple code.

Change-Id: If43a1f6fa607f07eded8511639898ea99a023cd6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2023358
Commit-Queue: Darin Fisher <darin@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736455}