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

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=jlebel@chromium.org

Bug: 1042208
Change-Id: I9890a8a1f34b406148f8f6c37d22a6839b03a5db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007727
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733597}

ios/chrome/browser/unified_consent/unified_consent_service_factory.h

@@ -8,13 +8,11 @@
 #include "base/macros.h"
 #include "base/no_destructor.h"
 #include "components/keyed_service/ios/browser_state_keyed_service_factory.h"
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
 
 namespace unified_consent {
 class UnifiedConsentService;
 }
-namespace ios {
-class ChromeBrowserState;
-}
 
 class UnifiedConsentServiceFactory : public BrowserStateKeyedServiceFactory {
  public: