back nudge: fix the issue that nudge can be fired more than once.

For one navigation operation, we would like the nudge only be fired
once. So this CL makes more restriction on when the navigation status
change can be fired to show the nudge ui (and cancel the previous one if
applicable).

Bug: 1009005b
Change-Id: I90b79e9ffb6219a66172364c976e69a9479670dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2086553Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746830}

chrome/browser/ui/ash/back_gesture_contextual_nudge_delegate.cc

@@ -46,8 +46,14 @@ void BackGestureContextualNudgeDelegate::MaybeStartTrackingNavigation(
 void BackGestureContextualNudgeDelegate::DidFinishNavigation(
     content::NavigationHandle* navigation_handle) {
   DCHECK(window_);
-  if (navigation_handle->HasCommitted())
+  // Make sure for one valid navigation, we only fire one status change
+  // notification.
+  if (navigation_handle->HasCommitted() &&
+      (navigation_handle->IsInMainFrame() ||
+       navigation_handle->HasSubframeNavigationEntryCommitted()) &&
+      (navigation_handle->GetURL() != navigation_handle->GetPreviousURL())) {
     controller_->NavigationEntryChanged(window_);
+  }
 }
 
 void BackGestureContextualNudgeDelegate::OnTabStripModelChanged(