Bug 1489302 - Follow-up from Bug 1488084. Restore accidentally removed sourceDoc. r=IanN

suite/mailnews/mailWindow.js

@@ -67,9 +67,13 @@ function OnMailWindowUnload()
  */
 function onCopyOrDragStart(e) {
   let browser = getBrowser();
-  if (!browser || e.target.ownerDocument != browser.contentDocument) {
+  if (!browser) {
+    return;
+  }
+  let sourceDoc = browser.contentDocument;
+  if (e.target.ownerDocument != sourceDoc) {
     // We're only interested if this is in the message content.
-    return; 
+    return;
   }
 
   let imgMap = new Map(); // Mapping img.src -> dataURL.