Bug 1147487 - don't bother sending reader mode updates when isArticle is false, r=margaret

--HG--
extra : rebase_source : ccc17edf45f1cc70d9582ecd1a30db737deb3c14

browser/base/content/content.js

@@ -568,11 +568,16 @@ let AboutReaderListener = {
     }
   },
   updateReaderButton: function() {
-    if (!ReaderMode.isEnabledForParseOnLoad || this.isAboutReader) {
+    if (!ReaderMode.isEnabledForParseOnLoad || this.isAboutReader ||
+        !(content.document instanceof content.HTMLDocument) ||
+        content.document.mozSyntheticDocument) {
       return;
     }
-    let isArticle = ReaderMode.isProbablyReaderable(content.document);
-    sendAsyncMessage("Reader:UpdateReaderButton", { isArticle: isArticle });
+    // Only send updates when there are articles; there's no point updating with
+    // |false| all the time.
+    if (ReaderMode.isProbablyReaderable(content.document)) {
+      sendAsyncMessage("Reader:UpdateReaderButton", { isArticle: true });
+    }
   },
 };
 AboutReaderListener.init();