MD User Manager: Refactors existing dialogs in User Manager into <user-manager-dialog>

chrome/browser/resources/md_user_manager/user_manager_styles.html

 <link rel="import" href="chrome://md-user-manager/shared_styles.html">
 
 <dom-module id="user-manager-styles">
   <template>
     <style include="shared-styles">
       body {
         background-color: var(--paper-grey-100);
       }
 
       user-manager-pages,
@@ skipping 214 matching lines @@
       .action-box-menu-title-name,
       .action-box-menu-title-email {
         height: auto;
       }
 
       .action-box-menu-title-email {
         color: var(--paper-grey-600);
       }
 
       .action-box-menu-remove {
-        border-top: 1px solid rgba(0, 0, 0, .12);
+        border-top: var(--user-manager-separator-line);
         line-height: 32px;
         padding: 8px 12px;
       }
 
       .action-box-remove-user-warning {
         align-items: center;
-        border-top: 1px solid rgba(0, 0, 0, .12);
+        border-top: var(--user-manager-separator-line);
         display: flex;
         flex-direction: column;
         font-size: inherit;
         line-height: 20px;
         padding: 12px;
       }
 
       .action-box-remove-user-warning > * {
         word-wrap: break-word;
       }
@@ skipping 10 matching lines @@
           font-weight: 500;
         };
         --paper-button-flat-keyboard-focus: {
           background: rgb(173, 50, 36);
         };
         @apply(--action-button);
       }
     </style>
   </template>
 </dom-module>