WebView Prerender: Support CancellationSignal for startPrerendering()

This CL supports CancellationSignal for AwContents.startPrerendering().

To identify which prerendering should be canceled, this CL introduces
the handle id (prerender id) in PrerenderHandle. StartPrerendering()
returns an id when prerendering successfully started. A caller of the
function can cancel prerendering by calling cancelPrerendering() with
the id.

CancellationSignal.OnCancelListener.onCancel() is fired when
CancellationSignal.cancel() is called. The listener takes the handle id
on prerendering start, and calls cancelPrerendering() with the id when
cancel() is called.

Bug: 41490450
Change-Id: Ifebb28089001aa1d02961ff0e7825d4da2452bdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6204842
Reviewed-by: Taiyo Mizuhashi <taiyo@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Lingqi Chi <lingqi@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Richard (Torne) Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1412757}