[ios] Fix memory leak for each tab

CRWWebViewScrollViewProxy passes itself to its delegate,
CRWWebViewScrollViewDelegateProxy, and both were keeping strong
pointers to each other.

Change-Id: Icb13eddcbb4cce14a2691c8b25fe5708cde6f699
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2041628Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: David Jean <djean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739355}

ios/web/web_state/ui/crw_web_view_scroll_view_delegate_proxy.mm

@@ -14,7 +14,7 @@
 
 @interface CRWWebViewScrollViewDelegateProxy ()
 
-@property(nonatomic) CRWWebViewScrollViewProxy* scrollViewProxy;
+@property(nonatomic, weak) CRWWebViewScrollViewProxy* scrollViewProxy;
 
 @end