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At the time OnNavigationStarted() is called it's not safe to call
Navigate(). This patch delays the navigate until it is safe (when
NavigationThrottleImpl::WillStartRequest() is called).

This is a reland because of a use-after-free. In particular, when
NavigationThrottle calls out to DoNavigate, navigationThrottle may
be deleted. The code was not set up to handle that.

BUG=1070569
TEST=NavigationBrowserTest.NavigateFromRendererInitiatedNavigation

Change-Id: I769ef0e0143d333e2fddbd997be2dc9de7aba3bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2154592Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760102}