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

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=dvallet@chromium.org

Bug: 1042208
Change-Id: Ic014ac6c3dc2cdaec2cbf7a088a32d7d6ce529f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007423
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: David Vallet <dvallet@chromium.org>
Commit-Queue: David Vallet <dvallet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733535}

ios/chrome/browser/language/language_model_manager_factory.h

@@ -11,10 +11,7 @@
 #include "base/no_destructor.h"
 #include "components/keyed_service/core/keyed_service.h"
 #include "components/keyed_service/ios/browser_state_keyed_service_factory.h"
-
-namespace ios {
-class ChromeBrowserState;
-}
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
 
 namespace language {
 class LanguageModelManager;

ios/chrome/browser/language/url_language_histogram_factory.h

@@ -11,10 +11,7 @@
 #include "base/no_destructor.h"
 #include "components/keyed_service/core/keyed_service.h"
 #include "components/keyed_service/ios/browser_state_keyed_service_factory.h"
-
-namespace ios {
-class ChromeBrowserState;
-}
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
 
 namespace language {
 class UrlLanguageHistogram;