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

chrome/browser/resources/md_history/synced_device_manager.js

Index: chrome/browser/resources/md_history/synced_device_manager.js
diff --git a/chrome/browser/resources/md_history/synced_device_manager.js b/chrome/browser/resources/md_history/synced_device_manager.js
index d329273b540af8784d9b8d819777055b41e6feff..517c913d7399e357c2a04ea246a5e311934ca8cd 100644
--- a/chrome/browser/resources/md_history/synced_device_manager.js
+++ b/chrome/browser/resources/md_history/synced_device_manager.js
@@ -39,9 +39,11 @@ Polymer({
     },
 
     /** @private */
-    signInState_: {
+    signInState: {
       type: Boolean,
+      // Updated on attach by chrome.sending 'otherDevicesInitialized'.
       value: loadTimeData.getBoolean('isUserSignedIn'),
+      observer: 'signInStateChanged_',
     },
 
     /** @private */
@@ -218,18 +220,13 @@ Polymer({
    * 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;
     }