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This CL moves the ownership of NavigationRequests from a map in NavigatorImpl
to the FrameTreeNode. This ensures that NavigationRequests are unique per
FrameTreeNode and are also properly deleted on destruction of the
FrameTreeNode. It also makes it easier to have PlzNavigate adapt to the
refactoring of DidStart/StopLoading happening in
https://codereview.chromium.org/1080143003/.

BUG=439423

Review URL: https://codereview.chromium.org/1080073004

Cr-Commit-Position: refs/heads/master@{#325498}