Zero-out crw_web_controller's container view delegate in dealloc.

Exposes delegate setter on CRWWebControllerContainerView and sets
it to zero in CRWWebController dealloc.

BUG=None
TEST=None

Review-Url: https://codereview.chromium.org/2453473002
Cr-Commit-Position: refs/heads/master@{#430253}

ios/web/web_state/ui/crw_web_controller.mm

@@ -1135,6 +1135,8 @@ const NSTimeInterval kSnapshotOverlayTransition = 0.5;
   DCHECK([NSThread isMainThread]);
   DCHECK(_isBeingDestroyed);  // 'close' must have been called already.
   DCHECK(!_webView);
+  // TODO(crbug.com/662860): Don't set the delegate to nil.
+  [_containerView setDelegate:nil];
   _touchTrackingRecognizer.get().touchTrackingDelegate = nil;
   [[_webViewProxy scrollViewProxy] removeObserver:self];
   [[NSNotificationCenter defaultCenter] removeObserver:self];

ios/web/web_state/ui/crw_web_controller_container_view.h

@@ -39,6 +39,8 @@
 @property(nonatomic, retain, readonly) id<CRWNativeContent> nativeController;
 // The currently displayed transient content view.
 @property(nonatomic, retain, readonly) CRWContentView* transientContentView;
+@property(nonatomic, assign) id<CRWWebControllerContainerViewDelegate>
+    delegate;  // weak
 
 // Designated initializer.  |proxy|'s content view will be updated as different
 // content is added to the container.

ios/web/web_state/ui/crw_web_controller_container_view.mm

@@ -212,6 +212,14 @@
                                UIEdgeInsetsMake(headerHeight, 0, 0, 0));
 }
 
+- (id<CRWWebControllerContainerViewDelegate>)delegate {
+  return _delegate.get();
+}
+
+- (void)setDelegate:(id<CRWWebControllerContainerViewDelegate>)delegate {
+  _delegate.reset(delegate);
+}
+
 #pragma mark Layout
 
 - (void)layoutSubviews {