Accessibility fixes for quick unlock settings.

chrome/browser/resources/settings/people_page/setup_pin_dialog.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="/icons.html">
 <link rel="import" href="/people_page/pin_keyboard.html">
 <link rel="import" href="/settings_shared_css.html">
 
 <dom-module id="settings-setup-pin-dialog">
   <template>
     <style include="settings-shared"></style>
     <style>
       .warning {
-        color: var(--paper-grey-500);
+        color: var(--paper-grey-700);
       }
 
       .warning > iron-icon {
-        --iron-icon-fill-color: var(--paper-grey-500);
+        --iron-icon-fill-color: var(--paper-grey-700);
       }
 
       .error {
         color: var(--paper-red-500);
       }
 
       .error > iron-icon {
         --iron-icon-fill-color: var(--paper-red-500);
       }
 
@@ skipping 29 matching lines @@
                         disabled$="[[!enableSubmit_]]">
             <span>[[getContinueMessage_(isConfirmStep_)]]</span>
           </paper-button>
         </div>
       </div>
     </dialog>
   </template>
 
   <script src="setup_pin_dialog.js"></script>
 </dom-module>