[ios] Use chrome_browser_state_forward.h in /ios/chrome/app/application_delegate

Instead of forward declaring ChromeBrowserState in the ios
namespace, use a header. This will allow moving the class
to the global namespace without having to change all the
forward declaration at the same time.

This CL was uploaded by git cl split.

R=marq@chromium.org

Bug: 1042208
Change-Id: I0c71b5c3173df9e89d8f616f2d02a844c99880c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007760
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733384}

ios/chrome/app/application_delegate/app_navigation.h

@@ -8,10 +8,7 @@
 #import <Foundation/Foundation.h>
 
 #include "base/ios/block_types.h"
-
-namespace ios {
-class ChromeBrowserState;
-}  // namespace ios
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
 
 @class SettingsNavigationController;