Speculative fix for ui::DisplayLinkMac::StopDisplayLink crashes

ui/accelerated_widget_mac/display_link_mac.cc

Index: ui/accelerated_widget_mac/display_link_mac.cc
diff --git a/ui/accelerated_widget_mac/display_link_mac.cc b/ui/accelerated_widget_mac/display_link_mac.cc
index d9cc46876f9f53a8aea2e04a668421efe6f5ffd5..8c9072717672b9abf58670011f26879e3a1ab904 100644
--- a/ui/accelerated_widget_mac/display_link_mac.cc
+++ b/ui/accelerated_widget_mac/display_link_mac.cc
@@ -86,6 +86,13 @@ DisplayLinkMac::DisplayLinkMac(
 DisplayLinkMac::~DisplayLinkMac() {
   StopDisplayLink();
 
+  // Usually |display_link_| holds the last reference to CVDisplayLinkRef, but
+  // that's not guaranteed, so it might not free all resources after the
+  // destructor completes. Ensure the callback is cleared out regardless.

[ccameron, 2016/01/25 18:04:17]

Maybe reference a bug # for the crashes?

[tapted, 2016/01/26 23:36:49]

Done.

+  CVReturn ret =
+      CVDisplayLinkSetOutputCallback(display_link_, nullptr, nullptr);
+  DCHECK_EQ(kCGErrorSuccess, ret);
+
   DisplayMap::iterator found = display_map_.Get().find(display_id_);
   DCHECK(found != display_map_.Get().end());
   DCHECK(found->second == this);