Bug 1588745 - part 4: Make TextControlState reuse its instance by itself r=Ehsan

Currently, only `HTMLInputElement` reuses `TextControlState` instance since
`HTMLTextAreaElement` had the instance as a member rather than allocate it.

Now, all instances are allocated in the heap independently for guaranteeing
their lifetime.  So, the reuse mechanism should be managed by
`TextControlState` itself.

Depends on D51393

Differential Revision: https://phabricator.services.mozilla.com/D51394

dom/html/HTMLInputElement.h

@@ -846,8 +846,6 @@ class HTMLInputElement final : public nsGenericHTMLFormElementWithState,
   void UpdateEntries(
       const nsTArray<OwningFileOrDirectory>& aFilesOrDirectories);
 
-  static void Shutdown();
-
   /**
    * Returns if the required attribute applies for the current type.
    */
@@ -1660,11 +1658,6 @@ class HTMLInputElement final : public nsGenericHTMLFormElementWithState,
     nsCOMPtr<nsIFilePicker> mFilePicker;
     RefPtr<HTMLInputElement> mInput;
   };
-
-  static void ReleaseTextControlState(TextControlState* aState);
-
-  static TextControlState* sCachedTextControlState;
-  static bool sShutdown;
 };
 
 }  // namespace dom