Preloading: Reset PreloadingAttempt inside prerender_host

We see PreloadingTriggeringOutcome::kFailure to kReady transition.
This can happen when we use the same PreloadingAttempt for different
navigation inside prerender frame tree setting TriggeringOutcome from kFailure to kReady in DidFinishNavigation.

The reason is that we don't reset the PreloadingAttempt and wait till the destruction of PrerenderHost, which happens async.

Now to fix this, we reset the PreloadingAttempt once failed. We also
re enable the flaky tests.

This might be one of the reasons we are seeing a lot of kReady states
for accurate triggering prerenders.

Bug: 1350870, 1352626, 1348636, 1361590
Change-Id: Iddb0094a00e72f25dad8fa76eaf812286e3d15f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3877736


Reviewed-by: Simon Pelchat <spelchat@chromium.org>
Commit-Queue: Sreeja Kamishetty <sreejakshetty@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1045346}