[ios] Fix uses of ios::ChromeBrowserState in /ios/chrome/browser/overlays

The ChromeBrowserState has been moved from the "ios" namespace
to the global namespace by http://crrev.com/c/2014913. Fix the
uses from ios::ChromeBrowserState to ChromeBrowserState.

Also remove includes of chrome_browser_state_forward.h and use
a simple forward declaration instead (since the old name is no
longer necessary).

This CL was uploaded by git cl split.

R=noyau@chromium.org

Bug: 1042208
Change-Id: I9e5c5b84cacf0fceb728dd282b0de6aeb9d066eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2017504Reviewed-by: Eric Noyau <noyau@chromium.org>
Commit-Queue: Eric Noyau <noyau@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735840}

ios/chrome/browser/overlays/overlay_browser_agent_base_unittest.mm

@@ -110,7 +110,7 @@ class OverlayBrowserAgentBaseTest : public PlatformTest {
 
  protected:
   web::WebTaskEnvironment task_environment_;
-  std::unique_ptr<ios::ChromeBrowserState> browser_state_;
+  std::unique_ptr<ChromeBrowserState> browser_state_;
   FakeWebStateListDelegate web_state_list_delegate_;
   WebStateList web_state_list_;
   web::WebState* web_state_ = nullptr;