[MD History] Add UMA stats for switching views and the CBD button.

chrome/browser/resources/md_history/synced_device_manager.js

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @typedef {{device: string,
  *           lastUpdateTime: string,
  *           separatorIndexes: !Array<number>,
  *           timestamp: number,
  *           tabs: !Array<!ForeignSessionTab>,
@@ skipping 21 matching lines @@
     /**
      * An array of synced devices with synced tab data.
      * @type {!Array<!ForeignDeviceInternal>}
      */
     syncedDevices_: {
       type: Array,
       value: function() { return []; }
     },
 
     /** @private */
-    signInState_: {
+    signInState: {
       type: Boolean,
+      // Updated on attach by chrome.sending 'otherDevicesInitialized'.
       value: loadTimeData.getBoolean('isUserSignedIn'),
+      observer: 'signInStateChanged_',
     },
 
     /** @private */
     guestSession_: {
       type: Boolean,
       value: loadTimeData.getBoolean('isGuestSession'),
     },
 
     /** @private */
     fetchingSyncedTabs_: {
@@ skipping 156 matching lines @@
    */
   tabSyncDisabled: function() {
     this.fetchingSyncedTabs_ = false;
     this.clearDisplayedSyncedDevices_();
   },
 
   /**
    * Get called when user's sign in state changes, this will affect UI of synced
    * tabs page. Sign in promo gets displayed when user is signed out, and
    * different messages are shown when there are no synced tabs.
-   * @param {boolean} isUserSignedIn
+   * @param {boolean} signInState
    */
-  updateSignInState: function(isUserSignedIn) {
-    // If user's sign in state didn't change, then don't change message or
-    // update UI.
-    if (this.signInState_ == isUserSignedIn)
-      return;
-
-    this.signInState_ = isUserSignedIn;
+  signInStateChanged_: function(signInState) {
+    this.fire('history-view-changed');
 
     // User signed out, clear synced device list and show the sign in promo.
-    if (!isUserSignedIn) {
+    if (!signInState) {
       this.clearDisplayedSyncedDevices_();
       return;
     }
     // User signed in, show the loading message when querying for synced
     // devices.
     this.fetchingSyncedTabs_ = true;
   },
 
   searchTermChanged: function(searchTerm) {
     this.clearDisplayedSyncedDevices_();
     this.updateSyncedDevices(this.sessionList);
   }
 });