MD-Settings: Adds an a11y label to most toggle buttons.

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

 <link rel="import" href="chrome://resources/cr_elements/cr_slider/cr_slider.html">
 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/html/md_select_css.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/classes/iron-flex-layout.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-tabs/paper-tabs.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-toggle-button/paper-toggle-button.html">
 <link rel="import" href="display_layout.html">
@@ skipping 68 matching lines @@
         </template>
       </paper-tabs>
     </div>
     <div hidden="[[hasMultipleDisplays_(displays)]]"
         class="settings-box line-only"></div>
     <div class="settings-box layout vertical first">
       <h2>[[selectedDisplay.name]]</h2>
       <div class="settings-box embedded first two-line"
           hidden$="[[!showMirror_(displays)]]">
         <div class="start">
-          <div>$i18n{displayMirror}</div>
+          <div id="displayMirrorCheckboxLabel">$i18n{displayMirror}</div>
           <div class="secondary">[[getDisplayMirrorText_(displays)]]</div>
         </div>
         <paper-toggle-button checked="[[isMirrored_(displays)]]"
-            on-tap="onMirroredTap_">
+            on-tap="onMirroredTap_"
+            aria-labelledby="displayMirrorCheckboxLabel">
       </div>
       <div class="settings-box embedded"
           hidden$="[[!showDisplaySelectMenu_(displays, selectedDisplay)]]">
         <div class="start">$i18n{displayScreenTitle}</div>
         <div class="md-select-wrapper">
           <select class="md-select" on-change="updatePrimaryDisplay_"
               value="[[getDisplaySelectMenuIndex_(
                   selectedDisplay, primaryDisplayId)]]">
             <option value="0">$i18n{displayScreenPrimary}</option>
             <option value="1">$i18n{displayScreenExtended}</option>
@@ skipping 55 matching lines @@
           </div>
         </div>
         <button class="subpage-arrow" is="paper-icon-button-light"
             aria-label="$i18n{displayTouchCalibrationTitle}"
             aria-describedby="touchCalibrationSecondary"></button>
       </div>
     </div>
   </template>
   <script src="display.js"></script>
 </dom-module>