Bug 1630208 part 3: Clean up the remote XPC document accessible cache on shutdown. r=yzen

The cache should already be empty by this point unless there's a bug, but we should still clean up regardless.
Also, the hashtable itself was previously never cleaned up even if it was empty.

Differential Revision: https://phabricator.services.mozilla.com/D79781

accessible/base/DocManager.cpp

@@ -40,9 +40,9 @@ using namespace mozilla::a11y;
 using namespace mozilla::dom;
 
 StaticAutoPtr<nsTArray<DocAccessibleParent*>> DocManager::sRemoteDocuments;
-nsRefPtrHashtable<nsPtrHashKey<const DocAccessibleParent>,
-                  xpcAccessibleDocument>* DocManager::sRemoteXPCDocumentCache =
-    nullptr;
+StaticAutoPtr<nsRefPtrHashtable<nsPtrHashKey<const DocAccessibleParent>,
+                                xpcAccessibleDocument>>
+    DocManager::sRemoteXPCDocumentCache;
 
 ////////////////////////////////////////////////////////////////////////////////
 // DocManager
@@ -143,6 +143,7 @@ xpcAccessibleDocument* DocManager::GetXPCDocument(DocAccessibleParent* aDoc) {
     sRemoteXPCDocumentCache =
         new nsRefPtrHashtable<nsPtrHashKey<const DocAccessibleParent>,
                               xpcAccessibleDocument>;
+    ClearOnShutdown(&sRemoteXPCDocumentCache);
   }
 
   MOZ_ASSERT(!aDoc->IsShutdown(), "Adding a shutdown doc to remote XPC cache");

accessible/base/DocManager.h

@@ -172,8 +172,9 @@ class DocManager : public nsIWebProgressListener,
                             xpcAccessibleDocument>
       XPCDocumentHashtable;
   XPCDocumentHashtable mXPCDocumentCache;
-  static nsRefPtrHashtable<nsPtrHashKey<const DocAccessibleParent>,
-                           xpcAccessibleDocument>* sRemoteXPCDocumentCache;
+  static StaticAutoPtr<nsRefPtrHashtable<
+      nsPtrHashKey<const DocAccessibleParent>, xpcAccessibleDocument>>
+      sRemoteXPCDocumentCache;
 
   /*
    * The list of remote top level documents.