[ios] Purge web view if external app URL is current in navigation stack.

ios/web/web_state/ui/crw_web_controller.mm

Index: ios/web/web_state/ui/crw_web_controller.mm
diff --git a/ios/web/web_state/ui/crw_web_controller.mm b/ios/web/web_state/ui/crw_web_controller.mm
index 6c926c06c7b6927f29403a948152e5f75f48c778..be0d8c44789dcac79a215e5d5686b6649ad04105 100644
--- a/ios/web/web_state/ui/crw_web_controller.mm
+++ b/ios/web/web_state/ui/crw_web_controller.mm
@@ -3343,6 +3343,17 @@ const NSTimeInterval kSnapshotOverlayTransition = 0.5;
     if ([self isMainFrameNavigationAction:action])
       [self stopLoading];
 
+    // Purge web view if last committed URL is different from the document URL.
+    // This can happen if external URL was added to the navigation stack and was
+    // loaded using Go Back or Go Forward navigation (in which case document URL
+    // will point to the previous page).
+    GURL lastCommittedURL =
+        self.webState->GetNavigationManager()->GetLastCommittedItem()->GetURL();
+    if (lastCommittedURL != _documentURL) {
+      [self requirePageReconstruction];
+      [self setDocumentURL:lastCommittedURL];
+    }
+
     if ([_delegate openExternalURL:requestURL
                        linkClicked:isNavigationTypeLinkActivated]) {
       // Record the URL so that errors reported following the 'NO' reply can be