Clip the BVC content to the BVC's view's bounds

The content of the WebView overflows its bounds to be displayed below
the toolbars (allowing to have a blur effect on the toolbars).
But during the rotation the content is displayed outside the bounds of
the main view, creating a weird effect.

This CL fixes it by clipping the content of the BVC to BVC's bounds.

Bug: 809103
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ica7a34fab6070a780b3e1eb03cae1ae2ad43e04b
Reviewed-on: https://chromium-review.googlesource.com/924150Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537764}

ios/chrome/browser/ui/browser_view_controller.mm

@@ -1574,6 +1574,11 @@ applicationCommandEndpoint:(id<ApplicationCommands>)applicationCommandEndpoint {
   UIViewAutoresizing initialViewAutoresizing =
       UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
 
+  // Clip the content to the bounds of the view. This prevents the WebView to
+  // overflow outside of the BVC, which is particularly visible during rotation.
+  // The WebView is overflowing its bounds to be displayed below the toolbars.
+  self.view.clipsToBounds = YES;
+
   self.contentArea =
       [[BrowserContainerView alloc] initWithFrame:initialViewsRect];
   self.contentArea.autoresizingMask = initialViewAutoresizing;