Implement Mac Pop-up Dictionary for OOPIF.

content/browser/renderer_host/text_input_client_mac.mm

Index: content/browser/renderer_host/text_input_client_mac.mm
diff --git a/content/browser/renderer_host/text_input_client_mac.mm b/content/browser/renderer_host/text_input_client_mac.mm
index a6183d87ee23f141723d58df10aa6f39c736e372..74f3dc2d74209d5d65a92b80757ea6086d73d724 100644
--- a/content/browser/renderer_host/text_input_client_mac.mm
+++ b/content/browser/renderer_host/text_input_client_mac.mm
@@ -37,6 +37,10 @@ void TextInputClientMac::GetStringAtPoint(
     RenderWidgetHost* rwh,
     gfx::Point point,
     void (^reply_handler)(NSAttributedString*, NSPoint)) {
+  // TODO(ekaramad): In principle, we are using the same handler regardless of
+  // the |rwh| which requested this. We should track the callbacks for each
+  // |rwh| individually so that one slow RWH will not end up clearing the
+  // callback for another (https://crbug.com/643233).
   DCHECK(replyForPointHandler_.get() == nil);
   replyForPointHandler_.reset(reply_handler, base::scoped_policy::RETAIN);
   RenderWidgetHostImpl* rwhi = RenderWidgetHostImpl::From(rwh);