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

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=edchin@chromium.org

Bug: 1042208
Change-Id: I21c2b6e63b6b329eb215bef2dc87b15f4fb872b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007580
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733726}