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This CL makes it so that an ongoing navigation is reset when a
ueser-initiated same-site or a cross-site navigation commits. This
matches what we're doing when deleting the speculative RFH. This solves
the issue with
WebContentsImplTest.CrossSiteNavigationBackOldNavigationIgnored when
browser-side navigation is enabled: a navigation commits and deletes the
speculative RFH created by another navigation, which is unexpected from
the second NavigationRequest. With this fix, the NavigationRequest is
reset at the same time as the speculative RFH.

BUG=439423
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2368533002
Cr-Commit-Position: refs/heads/master@{#420783}