[ios] Use chrome_browser_state_forward.h in /ios/chrome/browser/device_sharing

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=erikchen@chromium.org

Bug: 1042208
Change-Id: I9148cc5a2f2ecdfa4d081e27470660b22604f431
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007640
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734390}

ios/chrome/browser/device_sharing/device_sharing_manager.h

@@ -7,13 +7,11 @@
 
 #import <Foundation/Foundation.h>
 
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
+
 class GURL;
 @class HandoffManager;
 
-namespace ios {
-class ChromeBrowserState;
-}
-
 // This manager maintains all state related to sharing the active URL to other
 // devices. It has the role of a dispatcher that shares the active URL to
 // various internal sharing services (e.g. handoff).