Make a Reader Mode pageAction button

reader-mode-page-action.diff

diff --git a/browser/actors/AboutReaderParent.jsm b/browser/actors/AboutReaderParent.jsm
--- a/browser/actors/AboutReaderParent.jsm
+++ b/browser/actors/AboutReaderParent.jsm
@@ -14,16 +14,21 @@ ChromeUtils.defineModuleGetter(
   "PlacesUtils",
   "resource://gre/modules/PlacesUtils.jsm"
 );
 ChromeUtils.defineModuleGetter(
   this,
   "ReaderMode",
   "resource://gre/modules/ReaderMode.jsm"
 );
+ChromeUtils.defineModuleGetter(
+  this,
+  "PageActions",
+  "resource:///modules/PageActions.jsm"
+);
 
 const gStringBundle = Services.strings.createBundle(
   "chrome://global/locale/aboutReader.properties"
 );
 
 // A set of all of the AboutReaderParent actors that exist.
 // See bug 1631146 for a request for a less manual way of doing this.
 let gAllActors = new Set();
@@ -177,16 +182,17 @@ class AboutReaderParent extends JSWindow
       return;
     }
 
     let win = browser.ownerGlobal;
 
     let button = win.document.getElementById("reader-mode-button");
     let menuitem = win.document.getElementById("menu_readerModeItem");
     let key = win.document.getElementById("key_toggleReaderMode");
+    PageActions.actionForID("readerMode").setDisabled(!browser.isArticle, win);
     if (this.isReaderMode()) {
       gAllActors.add(this);
 
       let closeText = gStringBundle.GetStringFromName("readerView.close");
 
       button.setAttribute("readeractive", true);
       button.hidden = false;
       button.setAttribute("aria-label", closeText);
diff --git a/browser/modules/PageActions.jsm b/browser/modules/PageActions.jsm
--- a/browser/modules/PageActions.jsm
+++ b/browser/modules/PageActions.jsm
@@ -31,16 +31,21 @@ ChromeUtils.defineModuleGetter(
   "BinarySearch",
   "resource://gre/modules/BinarySearch.jsm"
 );
 ChromeUtils.defineModuleGetter(
   this,
   "PrivateBrowsingUtils",
   "resource://gre/modules/PrivateBrowsingUtils.jsm"
 );
+ChromeUtils.defineModuleGetter(
+  this,
+  "AboutReaderParent",
+  "resource:///actors/AboutReaderParent.jsm"
+);
 
 const ACTION_ID_BOOKMARK = "bookmark";
 const ACTION_ID_PIN_TAB = "pinTab";
 const ACTION_ID_BOOKMARK_SEPARATOR = "bookmarkSeparator";
 const ACTION_ID_BUILT_IN_SEPARATOR = "builtInSeparator";
 const ACTION_ID_TRANSIENT_SEPARATOR = "transientSeparator";
 
 const PREF_PERSISTED_ACTIONS = "browser.pageActions.persistedActions";
@@ -1168,16 +1173,24 @@ var gBuiltInActions = [
     onShowingInPanel(buttonNode) {
       browserPageActions(buttonNode).bookmark.onShowingInPanel(buttonNode);
     },
     onCommand(event, buttonNode) {
       browserPageActions(buttonNode).bookmark.onCommand(event, buttonNode);
     },
   },
 
+  // reader mode
+  {
+    id: "readerMode",
+    onCommand(event, buttonNode) {
+      AboutReaderParent.buttonClick(event);
+    },
+  },
+
   // pin tab
   {
     id: ACTION_ID_PIN_TAB,
     onBeforePlacedInWindow(browserWindow) {
       function handlePinEvent() {
         browserPageActions(browserWindow).pinTab.updateState();
       }
       function handleWindowUnload() {