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This is the 3rd CL of multiple CLs to remove the usages of
NavigationHandleImpl completely and replace it with NavigationRequest.

This CL replaces NavigationHandleImpl in MixedContentNavigationThrottle
and NavigationHandleImplBrowserTest with NavigationRequest, and renames
NavigationHandleImplBrowserTest to NavigationRequestBrowserTest.

Bug: 995268
Change-Id: Ia0d76dda052f4cea67586cafdc51635a4a3c8a1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1803447
Commit-Queue: Mohamed Abdelhalim <zetamoo@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697560}