MD History: Fix broken sync status dropdown

The sync (i) icon in the top-right of the page uses the iron-dropdown
Polymer element to display its content. This element was implicitly
imported by cr-shared-menu, meaning that after cr-shared-menu was
removed (in crrev.com/440308) from the history page, the dropdown
stopped working.

This CL restores the import of iron-dropdown, fixing the dropdown.

BUG=658821
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2594303002
Cr-Commit-Position: refs/heads/master@{#440815}

chrome/browser/resources/md_history/lazy_load.html

@@ -3,6 +3,7 @@
 <link rel="import" href="chrome://resources/cr_elements/cr_action_menu/cr_action_menu.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_dialog/cr_dialog.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/app-layout/app-drawer/app-drawer.html">
+<link rel="import" href="chrome://resources/polymer/v1_0/iron-dropdown/iron-dropdown.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button-light.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-item/paper-item.html">

chrome/browser/resources/md_history/lazy_load.vulcanized.html

@@ -1196,6 +1196,60 @@ paper-icon-button.subpage-arrow {
     </div>
   </template>
 
+  </dom-module>
+<dom-module id="iron-overlay-backdrop" assetpath="chrome://resources/polymer/v1_0/iron-overlay-behavior/" css-build="shadow">
+
+  <template>
+    <style scope="iron-overlay-backdrop">:host {
+  position: fixed;
+        top: 0;
+        left: 0;
+        width: 100%;
+        height: 100%;
+        background-color: var(--iron-overlay-backdrop-background-color, #000);
+        opacity: 0;
+        transition: opacity 0.2s;
+        pointer-events: none;
+        ;
+}
+
+:host(.opened) {
+  opacity: var(--iron-overlay-backdrop-opacity, 0.6);
+        pointer-events: auto;
+        ;
+}
+
+</style>
+
+    <content></content>
+  </template>
+
+</dom-module>
+
+<script src="chrome://resources/polymer/v1_0/web-animations-js/web-animations-next-lite.min.js"></script>
+
+
+<dom-module id="iron-dropdown" assetpath="chrome://resources/polymer/v1_0/iron-dropdown/" css-build="shadow">
+  <template>
+    <style scope="iron-dropdown">:host {
+  position: fixed;
+}
+
+#contentWrapper ::content > * {
+  overflow: auto;
+}
+
+#contentWrapper.animating ::content > * {
+  overflow: hidden;
+}
+
+</style>
+
+    <div id="contentWrapper">
+      <content id="content" select=".dropdown-content"></content>
+    </div>
+  </template>
+
   </dom-module>
 <dom-module id="paper-tab" assetpath="chrome://resources/polymer/v1_0/paper-tabs/" css-build="shadow">
   <template>