Bug 1350960 - Release CustomElementReactionsStack in TabGroup thread when DocGroup is going away. r=billm

dom/base/DocGroup.cpp

@@ -47,6 +47,11 @@ DocGroup::DocGroup(TabGroup* aTabGroup, const nsACString& aKey)
 DocGroup::~DocGroup()
 {
   MOZ_ASSERT(mDocuments.IsEmpty());
+  if (!NS_IsMainThread()) {
+    nsIEventTarget* target = EventTargetFor(TaskCategory::Other);
+    NS_ProxyRelease(target, mReactionsStack.forget());
+  }
+
   mTabGroup->mDocGroups.RemoveEntry(mKey);
 }