[MD History] Add shift-selection to the history lists.

chrome/browser/resources/md_history/list_container.js

Index: chrome/browser/resources/md_history/list_container.js
diff --git a/chrome/browser/resources/md_history/list_container.js b/chrome/browser/resources/md_history/list_container.js
index 6cc6518d31bd4c0f1d2988c936e23d6967f80f10..0d1402d0bbd0c38561547a3f5fe82d734f96f310 100644
--- a/chrome/browser/resources/md_history/list_container.js
+++ b/chrome/browser/resources/md_history/list_container.js
@@ -86,6 +86,11 @@ Polymer({
     ]);
   },
 
+  /** @return {number} */
+  getSelectedItemCount: function() {
+    return this.getSelectedList_().selectedPaths.size;
+  },
+
   unselectAllItems: function(count) {
     this.getSelectedList_().unselectAllItems(count);
   },