Speculative fix for ui::DisplayLinkMac::StopDisplayLink crashes

The CVDisplayLink is tied to a ScopedTypeRef data member of DisplayLinkMac.
However, there may be other references. Since there's no guarantee that
~DisplayLinkMac() will invoke the last CVDisplayLinkRelease(..) and clear
the callback automatically, clear it explicitly in ~DisplayLinkMac.

BUG=564780

Review URL: https://codereview.chromium.org/1626163002

Cr-Commit-Position: refs/heads/master@{#371648}

ui/accelerated_widget_mac/display_link_mac.cc

@@ -86,6 +86,14 @@ 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 to
+  // avoid possible crashes (see http://crbug.com/564780).
+  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);