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

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=noyau@chromium.org

Bug: 1042208
Change-Id: I7b90f7a9896a688b95adf8437677a4b544d791da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007586
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Eric Noyau <noyau@chromium.org>
Commit-Queue: Eric Noyau <noyau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733363}

ios/chrome/browser/dom_distiller/dom_distiller_service_factory.h

@@ -10,15 +10,12 @@
 #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"
 
 namespace dom_distiller {
 class DomDistillerService;
 }
 
-namespace ios {
-class ChromeBrowserState;
-}
-
 namespace dom_distiller {
 
 class DomDistillerServiceFactory : public BrowserStateKeyedServiceFactory {