MD Settings: Display: Add mirroring and other fixes

chrome/browser/resources/settings/device_page/display_layout.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-resizable-behavior/iron-resizable-behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="/device_page/drag_behavior.html">
 <link rel="import" href="/device_page/layout_behavior.html">
 <link rel="import" href="/settings_shared_css.html">
 
 <dom-module id="display-layout">
   <template>
     <style include="settings-shared">
       /* Use relative position with no offset so that display divs (children),
          which have absolute positions, are offset from the displayArea div. */
       #displayArea {
         border: lightgrey solid 1px;
         height: 100%;
         position: relative;
         width: 100%;
       }
+      /* Note: the size of the border / box shadow affects the style generated
+         in getDivStyle_ and getMirrorDivStyle_ */
       .display {
         align-items: center;
         background: var(--paper-grey-200);
         border: var(--paper-grey-200) solid 1px;
         box-shadow: 2px 2px var(--paper-grey-400);
         cursor: default;
         display: flex;
         justify-content: center;
         position: absolute;
       }
       .display[selected] {
         border: var(--google-blue-500) solid 2px;
         box-shadow: none;
       }
+      .display.mirror {
+        border: var(--google-blue-500) solid 1px;
+        box-shadow: none;
+      }
       .highlight-left {
         border-left: var(--google-blue-700) solid 1px;
       }
       .highlight-right {
         border-right: var(--google-blue-700) solid 1px;
       }
       .highlight-top {
         border-top: var(--google-blue-700) solid 1px;
       }
       .highlight-bottom {
         border-bottom: var(--google-blue-700) solid 1px;
       }
     </style>
     <div id="displayArea" on-iron-resize="calculateVisualScale_">
       <template is="dom-repeat" items="[[displays]]">
-        <div id="_[[item.id]]" class="display" draggable="true"
+        <div id="_mirror_[[item.id]]" class="display mirror"
+            hidden$="[[!mirroring]]"
+            style$="[[getMirrorDivStyle_(item.id, item.bounds, visualScale)]]"
+        </div>
+      </template>
+      <template is="dom-repeat" items="[[displays]]">
+        <div id="_[[item.id]]" class="display" draggable="[[dragEnabled]]"
             style$="[[getDivStyle_(item.id, item.bounds, visualScale)]]"
             selected$="[[isSelected_(item, selectedDisplay)]]"
             on-tap="onSelectDisplayTap_">
           [[item.name]]
         </div>
       </template>
     </div>
   </template>
   <script src="display_layout.js"></script>
 </dom-module>