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

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=olivierrobin@chromium.org

Bug: 1042208
Change-Id: I15d44cfb7e142a9c7fc41373d454ee997c7ed6fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2010784
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733391}

ios/chrome/browser/metrics/ios_chrome_metrics_service_client.h

@@ -21,16 +21,13 @@
 #include "components/omnibox/browser/omnibox_event_global_tracker.h"
 #include "components/ukm/observers/history_delete_observer.h"
 #include "components/ukm/observers/ukm_consent_state_observer.h"
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
 #import "ios/chrome/browser/metrics/incognito_web_state_observer.h"
 #include "ios/web/public/deprecated/global_web_state_observer.h"
 
 class IOSChromeStabilityMetricsProvider;
 class PrefRegistrySimple;
 
-namespace ios {
-class ChromeBrowserState;
-}
-
 namespace metrics {
 class MetricsService;
 class MetricsStateManager;

ios/chrome/browser/metrics/ios_profile_session_durations_service_factory.h

@@ -8,11 +8,9 @@
 #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"
 
 class IOSProfileSessionDurationsService;
-namespace ios {
-class ChromeBrowserState;
-}
 
 class IOSProfileSessionDurationsServiceFactory
     : public BrowserStateKeyedServiceFactory {

ios/chrome/browser/metrics/new_tab_page_uma.h

@@ -5,12 +5,9 @@
 #ifndef IOS_CHROME_BROWSER_METRICS_NEW_TAB_PAGE_UMA_H_
 #define IOS_CHROME_BROWSER_METRICS_NEW_TAB_PAGE_UMA_H_
 
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
 #include "ui/base/page_transition_types.h"
 
-namespace ios {
-class ChromeBrowserState;
-}  // namespace ios
-
 class GURL;
 namespace web {
 class WebState;