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

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=michaeldo@chromium.org

Bug: 1042208
Change-Id: I1d4ecec8dc16e169bca7ebd6697e16db57821d49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2017482
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734628}

ios/chrome/browser/ui/context_menu/context_menu_coordinator.h

@@ -34,8 +34,7 @@
 - (instancetype)initWithBaseViewController:(UIViewController*)viewController
     NS_UNAVAILABLE;
 - (instancetype)initWithBaseViewController:(UIViewController*)viewController
-                              browserState:
-                                  (ios::ChromeBrowserState*)browserState
+                              browserState:(ChromeBrowserState*)browserState
     NS_UNAVAILABLE;
 - (instancetype)initWithBaseViewController:(UIViewController*)viewController
                                    browser:(Browser*)browser NS_UNAVAILABLE;