[ios] Fixes a crash in -[CRWWebViewScrollViewDelegateProxy forwardInvocation:].

-[NSInvocation getArgument:atIndex:] fills its argument into a void* and
therefore doesn't trigger an ARC-synthesized retain. If the argument is
stored into a strong pointer, release is called when the pointer goes
out of scope, which causes the object to be overreleased. Annotating the
pointer as __unsafe_unretained avoids this problem.

BUG=1092875

Change-Id: I990c3878b9ab7d828ff17f3a3cf530059f7f5c85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2235656
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776508}

ios/web/web_state/ui/crw_web_view_scroll_view_delegate_proxy.mm

@@ -80,7 +80,7 @@
   if (signature.numberOfArguments >= 3 &&
       strcmp([signature getArgumentTypeAtIndex:2], @encode(UIScrollView*)) ==
           0) {
-    UIScrollView* sender;
+    __unsafe_unretained UIScrollView* sender;
     [invocation getArgument:&sender atIndex:2];
     if (sender == self.scrollViewProxy.underlyingScrollView) {
       sender = [self.scrollViewProxy asUIScrollView];