Bug 1349178 Make nsIEditor.transactionManager readonly r=m_kato

There are no app which replaces transaction manager of editor.  Additionally, replacing transaction manager of an editor with different editor or same editor but after initialized again due to some reasons may cause unexpected problems.

Therefore, this patch makes it readonly.

MozReview-Commit-ID: K7zepOjrzvC

editor/libeditor/EditorBase.cpp

@@ -787,16 +787,6 @@ EditorBase::GetTransactionManager(nsITransactionManager** aTxnManager)
   return NS_OK;
 }
 
-NS_IMETHODIMP
-EditorBase::SetTransactionManager(nsITransactionManager* aTxnManager)
-{
-  NS_ENSURE_TRUE(aTxnManager, NS_ERROR_FAILURE);
-
-  // nsITransactionManager is builtinclass, so this is safe
-  mTxnMgr = static_cast<nsTransactionManager*>(aTxnManager);
-  return NS_OK;
-}
-
 NS_IMETHODIMP
 EditorBase::Undo(uint32_t aCount)
 {

editor/nsIEditor.idl

@@ -172,7 +172,7 @@ interface nsIEditor  : nsISupports
 
   /** transactionManager Get the transaction manager the editor is using.
     */
-  attribute nsITransactionManager transactionManager;
+  readonly attribute nsITransactionManager transactionManager;
 
   /** doTransaction() fires a transaction.
     * It is provided here so clients can create their own transactions.