Bug 1342197 part 4. Move GetSelectionRange from nsTextControlFrame to the editor state. r=ehsan

At this point, all this method does is ensure editor initialization and then ask
the editor state for various information.  Let's cut out the middleman.

MozReview-Commit-ID: p491umScJO

dom/html/HTMLTextAreaElement.h

@@ -109,6 +109,9 @@ public:
   NS_IMETHOD_(void) InitializeKeyboardEventListeners() override;
   NS_IMETHOD_(void) OnValueChanged(bool aNotify, bool aWasInteractiveUserChange) override;
   NS_IMETHOD_(bool) HasCachedSelection() override;
+  NS_IMETHOD GetSelectionRange(int32_t* aSelectionStart,
+                               int32_t* aSelectionEnd) override;
+
 
   // nsIContent
   virtual nsresult BindToTree(nsIDocument* aDocument, nsIContent* aParent,
@@ -347,8 +350,6 @@ protected:
    */
   nsresult SetValueInternal(const nsAString& aValue, uint32_t aFlags);
 
-  nsresult GetSelectionRange(int32_t* aSelectionStart, int32_t* aSelectionEnd);
-
   /**
    * Common method to call from the various mutation observer methods.
    * aContent is a content node that's either the one that changed or its