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

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=droger@chromium.org

Bug: 1042208
Change-Id: I5fbed15d8a69cf472a24dcf0f855f617b0b9f62c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007814
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733378}

ios/chrome/browser/web_state_list/tab_insertion_browser_agent.h

@@ -7,12 +7,10 @@
 
 #import <Foundation/Foundation.h>
 
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
 #include "ios/chrome/browser/main/browser_user_data.h"
 #import "ios/web/public/navigation/navigation_manager.h"
 
-namespace ios {
-class ChromeBrowserState;
-}
 namespace web {
 class WebState;
 }

ios/chrome/browser/web_state_list/web_usage_enabler/BUILD.gn

@@ -17,6 +17,7 @@ source_set("web_usage_enabler") {
     "//base",
     "//components/keyed_service/core",
     "//components/keyed_service/ios",
+    "//ios/chrome/browser/browser_state:forward",
     "//ios/chrome/browser/web_state_list",
   ]
 }

ios/chrome/browser/web_state_list/web_usage_enabler/web_state_list_web_usage_enabler_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;
-}
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
 
 class WebStateListWebUsageEnabler;