[ios] Use chrome_browser_state_forward.h in /ios/public/provider/chrome/browser/signin

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.

Bug: 1042208
Change-Id: Ib442aa79586586b2495dec1df11497713ee7e442
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2011800
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734422}

ios/public/provider/chrome/browser/signin/BUILD.gn

@@ -20,6 +20,7 @@ source_set("signin") {
   deps = [
     "//base",
     "//google_apis",
+    "//ios/chrome/browser/browser_state:forward",
   ]
 }
 

ios/public/provider/chrome/browser/signin/chrome_identity_service.h

@@ -11,6 +11,7 @@
 
 #include "base/macros.h"
 #include "base/observer_list.h"
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
 
 @class ChromeIdentity;
 @protocol ChromeIdentityBrowserOpener;
@@ -29,7 +30,6 @@
 
 namespace ios {
 
-class ChromeBrowserState;
 class ChromeIdentityService;
 
 // Callback passed to method |GetAccessTokenForScopes()| that returns the