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

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=mahmadi@chromium.org

Bug: 1042208
Change-Id: I01d3540bee98e66b436e82135179f05dddc6ee5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007745
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733699}

ios/chrome/browser/ntp_tiles/ios_most_visited_sites_factory.h

@@ -7,9 +7,7 @@
 
 #include <memory>
 
-namespace ios {
-class ChromeBrowserState;
-}
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
 
 namespace ntp_tiles {
 class MostVisitedSites;

ios/chrome/browser/ntp_tiles/ios_popular_sites_factory.h

@@ -7,9 +7,7 @@
 
 #include <memory>
 
-namespace ios {
-class ChromeBrowserState;
-}
+#include "ios/chrome/browser/browser_state/chrome_browser_state_forward.h"
 
 namespace ntp_tiles {
 class PopularSites;