fix for #102753, use the disableonsend attribute trick for the subject

to be consistent.  r=varada, sr=bienvenu

mailnews/compose/resources/content/MsgComposeCommands.js

@@ -90,7 +90,6 @@ if (prefs) {
 function disableEditableFields()
 {
   editorShell.editor.SetFlags(plaintextEditor.eEditorReadonlyMask);
-  document.getElementById("msgSubject").setAttribute('disabled', 'true');
   var disableElements = document.getElementsByAttribute("disableonsend", "true");
   for (i=0;i<disableElements.length;i++)
   {
@@ -101,7 +100,6 @@ function disableEditableFields()
 function enableEditableFields()
 {
   editorShell.editor.SetFlags(plaintextEditor.eEditorMailMask);
-  document.getElementById("msgSubject").removeAttribute("disabled");
   var enableElements = document.getElementsByAttribute("disableonsend", "true");
   for (i=0;i<enableElements.length;i++)
   {

mailnews/compose/resources/content/messengercompose.xul

@@ -404,7 +404,7 @@
           <tree id="addressingWidgetTree" flex="1"/>
           <hbox autostretch="never">
             <text class="label" value="&subject.label;" accesskey="&subject.accesskey;"/>
-            <textbox id="msgSubject" flex="1" class="toolbar" onkeyup="SetComposeWindowTitle(event.which);"
+            <textbox id="msgSubject" flex="1" class="toolbar" onkeyup="SetComposeWindowTitle(event.which);" disableonsend="true"
                        onkeypress="subjectKeyPress(event);" onchange="contentChanged=true;"/>
           </hbox>
         </vbox>