[iOS] Prevent DCHECK when closing Find In Page/Text Zoom

The reason the DCHECK was being hit is because this forwarder wasn't
being properly cleaned up.

Fixed: 1044606
Change-Id: Iae58f7af86a6bc0af57a6f93756c0954fba43413
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2013132
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734066}

ios/chrome/browser/ui/find_bar/find_bar_mediator.mm

@@ -43,6 +43,10 @@
   return self;
 }
 
+- (void)dealloc {
+  _forwarder = nullptr;
+}
+
 #pragma mark - CRWWebStateObserver
 
 - (void)webState:(web::WebState*)webState

ios/chrome/browser/ui/text_zoom/text_zoom_mediator.mm

@@ -68,6 +68,8 @@
     _webStateList->RemoveObserver(_webStateListObserver.get());
     _webStateListObserver = nullptr;
     _webStateList = nullptr;
+
+    _forwarder = nullptr;
   }
 }