[ios] Use chrome_browser_state_forward.h in /ios/chrome/browser/ui/bubble

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=edchin@chromium.org

Bug: 1042208
Change-Id: Idc05dae62f02d8228a8d3bce4bc8cb68ad4ca0d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007582
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733782}

ios/chrome/browser/ui/bubble/bubble_presenter.h

@@ -7,9 +7,7 @@
 
 #import <UIKit/UIKit.h>
 
-namespace ios {
-class ChromeBrowserState;
-}
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
 
 @protocol BubblePresenterDelegate;
 @class BubbleViewControllerPresenter;