Bug 1397178 - Make EditorBase::GetRoot() not call nsIEditor::GetRootElement() r=smaug

In the old design, EditorBase::mRootElement is initialized when
nsIEditor::GetRootElement() is called.  Therefore, EditorBase::GetRoot()
calls if mRootElement is nullptr.

However, mRootElement is now initialized when EditorBase::UpdateRootElement()
is called and it's always initialized when EditorBase::Init() is called.

So, EditorBase::GetRoot() doesn't need to call nsIEditor::GetRootElement()
anymore.

MozReview-Commit-ID: 6dNEJaGNMZe

--HG--
extra : rebase_source : ed29488cfd1434fb200387706ca5a96cb2185090

editor/libeditor/EditorBase.h

@@ -972,16 +972,7 @@ public:
   /**
    * Fast non-refcounting editor root element accessor
    */
-  Element* GetRoot()
-  {
-    if (!mRootElement) {
-      // Let GetRootElement() do the work
-      nsCOMPtr<nsIDOMElement> root;
-      GetRootElement(getter_AddRefs(root));
-    }
-
-    return mRootElement;
-  }
+  Element* GetRoot() const { return mRootElement; }
 
   /**
    * Likewise, but gets the editor's root instead, which is different for HTML