Prerender: Support activation during ongoing main-frame navigation

This CL adds a new PrerenderCommitDeferringCondition to add
a mechanism to block activation until the ongoing main-frame
navigation commits.

We use WebContentsObserver to track the navigation to conditionally
resume the PrerenderCommitDeferringCondition after commit.

This work assumes no new navigations occur between the check inside
the PrerenderCommitDeferringCondition and the actual activation commit.

BUG= 1212656

Change-Id: I1b843d6ae4b1e65cd4f1fd25cfb5a027bf4d0e88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2837757


Commit-Queue: Sreeja Kamishetty <sreejakshetty@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#886219}