MD History: Replace last usage of cr-shared-menu with cr-action-menu.

ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.html

+<link rel="import" href="chrome://resources/cr_elements/shared_vars_css.html">

[dpapad, 2016/12/22 00:14:21]

This change causes a "Polymer is already defined." error during unit tests. I am
investigating...

[dpapad, 2016/12/22 00:46:18]

Fixed. It turns out that polymer.html must be at the top. This revealed a follow
up error which i fixed below.

 <link rel="import" href="chrome://resources/html/polymer.html">
 
 <dom-module id="cr-action-menu">
   <template>
     <style>
       :host {
         background-color: white;
         border: none;
         box-shadow: 0 2px 6px var(--paper-grey-500);
         margin: 0;
         outline: none;
         padding: 8px 0;
       }
 
       :host::backdrop {
         background-color: transparent;
       }
 
+      :host ::content .dropdown-item {
+        align-items: center;
+        background: none;
+        border: none;
+        box-sizing: border-box;
+        color: var(--paper-grey-800);
+        display: flex;
+        font: inherit;
+        min-height: 32px;
+        padding: 0 24px;
+        text-align: start;
+        width: 100%;
+      }
+
+      :host ::content .dropdown-item:not([disabled]) {

[dpapad, 2016/12/22 00:46:18]

Subtle bug that was caught by a test! hidden attribute was overriden by the
:host ::content .dropdown-item display: flex rule (which made a hidden option
not actually hidden), so I had to move it here.

+        @apply(--cr-actionable);
+      }
+
       :host ::content .dropdown-item:focus {
         background-color: var(--paper-grey-300);
         outline: none;
       }
     </style>
     <content select=".dropdown-item,hr"></content>
   </template>
   <script src="cr_action_menu.js"></script>
 </dom-module>