[fuchsia] Apply NavigationEventObserver FIDL rename to the file name.

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}

webrunner/BUILD.gn

@@ -214,7 +214,7 @@ fidl_library("fidl") {
     "fidl/context_provider.fidl",
     "fidl/frame.fidl",
     "fidl/navigation_controller.fidl",
-    "fidl/navigation_state_change_observer.fidl",
+    "fidl/navigation_event_observer.fidl",
   ]
 
   public_deps = [