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

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=gambard@chromium.org

Bug: 1042208
Change-Id: I0eb23bb95534e85d7177b871d7cf4ec1ad777973
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2017425Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735871}

ios/chrome/browser/ui/sad_tab/sad_tab_coordinator_unittest.mm

@@ -121,7 +121,7 @@ TEST_F(SadTabCoordinatorTest, FirstFailureInNonIncognito) {
 TEST_F(SadTabCoordinatorTest, FirstFailureInIncognito) {
   web::TestWebState web_state;
   web_state.WasShown();
-  ios::ChromeBrowserState* otr_browser_state =
+  ChromeBrowserState* otr_browser_state =
       browser_state_->GetOffTheRecordChromeBrowserState();
   SadTabCoordinator* coordinator = [[SadTabCoordinator alloc]
       initWithBaseViewController:base_view_controller_
@@ -144,7 +144,7 @@ TEST_F(SadTabCoordinatorTest, FirstFailureInIncognito) {
 
 // Tests SadTabViewController state for the repeated failure in incognito mode.
 TEST_F(SadTabCoordinatorTest, ShowFirstFailureInIncognito) {
-  ios::ChromeBrowserState* otr_browser_state =
+  ChromeBrowserState* otr_browser_state =
       browser_state_->GetOffTheRecordChromeBrowserState();
   SadTabCoordinator* coordinator = [[SadTabCoordinator alloc]
       initWithBaseViewController:base_view_controller_