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

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: Ic72a6611ab418566180ee72608e1115b1d1e9db6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007816
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@{#733731}

ios/chrome/browser/feature_engagement/tracker_factory.h

@@ -8,10 +8,7 @@
 #include "base/macros.h"
 #include "base/no_destructor.h"
 #include "components/keyed_service/ios/browser_state_keyed_service_factory.h"
-
-namespace ios {
-class ChromeBrowserState;
-}  // namespace ios
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
 
 namespace feature_engagement {
 class Tracker;

ios/chrome/browser/feature_engagement/tracker_util.h

@@ -5,9 +5,8 @@
 #ifndef IOS_CHROME_BROWSER_FEATURE_ENGAGEMENT_TRACKER_UTIL_H_
 #define IOS_CHROME_BROWSER_FEATURE_ENGAGEMENT_TRACKER_UTIL_H_
 
-namespace ios {
-class ChromeBrowserState;
-}  // namespace ios
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
+
 @class OpenNewTabCommand;
 
 namespace feature_engagement {