[iOS] Add touch recognizer back when WebUsage is enabled

When WebUsage is disabled, the touch recognizer is disconnected from the
WebController. The WebController is no longer notified of the touch
events.
The touch recognizer was never reconnected, even when WebUsage was
enabled back.
This CL completely removes the touch recognizer from the Container view
when the WebUsage is disabled, but add a new one when it is re-enabled.

Bug: 987613
Change-Id: I696955bf0d28ee08658f5e788afcb1ffc07cdc12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1724672
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683520}

ios/web/web_state/ui/crw_web_controller.mm

@@ -366,10 +366,17 @@ typedef void (^ViewportStateCompletion)(const web::PageViewportState*);
         setNativeControllerWebUsageEnabled:_webUsageEnabled];
     if (enabled) {
       // Don't create the web view; let it be lazy created as needed.
+
+      // The gesture is removed when the web usage is disabled. Add it back when
+      // it is enabled again.
+      [_containerView addGestureRecognizer:[self touchTrackingRecognizer]];
     } else {
       self.webStateImpl->ClearTransientContent();
-      _touchTrackingRecognizer.touchTrackingDelegate = nil;
-      _touchTrackingRecognizer = nil;
+      if (_touchTrackingRecognizer) {
+        [_containerView removeGestureRecognizer:_touchTrackingRecognizer];
+        _touchTrackingRecognizer.touchTrackingDelegate = nil;
+        _touchTrackingRecognizer = nil;
+      }
       _currentURLLoadWasTrigerred = NO;
     }
   }