Bug 1580462 - Follow-up: include a missing header file. r=baku

Differential Revision: https://phabricator.services.mozilla.com/D50435

--HG--
extra : moz-landing-system : lando

dom/base/DOMIntersectionObserver.cpp

@@ -11,6 +11,7 @@
 #include "nsLayoutUtils.h"
 #include "mozilla/PresShell.h"
 #include "mozilla/ServoBindings.h"
+#include "mozilla/dom/BrowsingContext.h"
 
 namespace mozilla {
 namespace dom {