Bug 1646996 - Proxy to main thread in ~DocGroup. r=smaug

When destroying the DocGroup, it will most certainly have no documents
attached to it, which makes labelling them a strange thing to do. And
the event target in DocGroup would most certainly dispatch to release
on the main thread anyway, so switch to do that right away.

Differential Revision: https://phabricator.services.mozilla.com/D88538

dom/base/DocGroup.cpp

@@ -186,11 +186,10 @@ DocGroup::~DocGroup() {
   MOZ_RELEASE_ASSERT(!mBrowsingContextGroup);
 
   if (!NS_IsMainThread()) {
-    nsIEventTarget* target = EventTargetFor(TaskCategory::Other);
-    NS_ProxyRelease("DocGroup::mReactionsStack", target,
-                    mReactionsStack.forget());
+    NS_ReleaseOnMainThread("DocGroup::mReactionsStack",
+                           mReactionsStack.forget());
 
-    NS_ProxyRelease("DocGroup::mArena", target, mArena.forget());
+    NS_ReleaseOnMainThread("DocGroup::mArena", mArena.forget());
   }
 
   if (mIframePostMessageQueue) {