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The NavigationStateChangeObserver interface was renamed to
NavigationEventObserver in a previous CL. This CL finishes up the
rename by changing the file name too.

Change-Id: I6e1d17d7aadceb1736541be6bf4b8f8c40d7e81e
Reviewed-on: https://chromium-review.googlesource.com/1217515Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590517}