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

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: I56c1cefb14ba115720b0d802c31e7453fa6bf865
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007761
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733357}

ios/chrome/browser/net/cookie_util.h

@@ -9,12 +9,9 @@
 
 #include "base/files/file_path.h"
 #include "base/memory/ref_counted.h"
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
 #include "net/cookies/canonical_cookie.h"
 
-namespace ios {
-class ChromeBrowserState;
-}
-
 namespace net {
 class CookieCryptoDelegate;
 class CookieStore;