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Calling into WebLocalFrame::WillStartNavigation may cancel the navigation,
for example when dispatched javascript event handlers do "window.stop()".

Currently, we call RFO::DidStartNavigation prematurely, and never issue a
paired "canceled" call when WillStartNavigation fails. We can instead try
to start and only then issue observer notification to ensure proper sequence.

Tests which navigate from onload handler now correctly report "first
navigation finished, second navigation started" instead of the reverse.

onreadystatechange-detach previously reported a new navigation, which does not
actually happen because frame detaches itself before being able to start
navigating. This behavior is fixed now.

Bug: 855189
Change-Id: Ia12518ebddf54557f4699cd39168217762c874eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1800916Reviewed-by: Camille Lamy <clamy@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697044}