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

chrome/test/data/webui/md_history/test_util.js

Index: chrome/test/data/webui/md_history/test_util.js
diff --git a/chrome/test/data/webui/md_history/test_util.js b/chrome/test/data/webui/md_history/test_util.js
index 0deeb3b54425f502e464344745c051eaf7ac753a..2aff411c096ae6574622df984bba39b271212481 100644
--- a/chrome/test/data/webui/md_history/test_util.js
+++ b/chrome/test/data/webui/md_history/test_util.js
@@ -111,6 +111,15 @@ function waitForEvent(element, eventName, predicate) {
 }
 
 /**
+ * @return {Promise}
+ */
+function waitForAnimationFrame() {
+  return new Promise(function(resolve) {
+    requestAnimationFrame(function() { setTimeout(resolve); });
+  });
+}
+
+/**
  * Sends a shift click event to |element|.
  * @param {HTMLElement} element
  */