MD History: Add spinners when new data is loading or searching

chrome/browser/resources/md_history/history_toolbar.js

Index: chrome/browser/resources/md_history/history_toolbar.js
diff --git a/chrome/browser/resources/md_history/history_toolbar.js b/chrome/browser/resources/md_history/history_toolbar.js
index 6c2dd401c5d3b18555ae1f9c0c53706fcaba72fd..9c579772e654567a69ec3d4f111920c1bb063c33 100644
--- a/chrome/browser/resources/md_history/history_toolbar.js
+++ b/chrome/browser/resources/md_history/history_toolbar.js
@@ -43,6 +43,12 @@ Polymer({
       value: ''
     },
 
+    // True if it's searching at the backend.
+    searching: {
+      type: Boolean,
+      value: false
+    },
+
     // True if the page has a sidebar that we need to account for when
     // positioning toolbar items.
     hasSidebar: {