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

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=olivierrobin@chromium.org

Bug: 1042208
Change-Id: I2379c1b78e2088aa448ab50b2156c99f545d8139
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2017363Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735362}

ios/chrome/browser/ui/qr_scanner/qr_scanner_legacy_coordinator.h

@@ -17,8 +17,7 @@
 
 // Unavailable, use -initWithBaseViewController:browser:.
 - (instancetype)initWithBaseViewController:(UIViewController*)viewController
-                              browserState:
-                                  (ios::ChromeBrowserState*)browserState
+                              browserState:(ChromeBrowserState*)browserState
     NS_UNAVAILABLE;
 
 @end