Hide scrollbars for the body of the sign-in dialogs.

chrome/browser/resources/signin/signin_shared_css.html

 <!-- Common styles for signin-related tab modal dialogs. -->
 <dom-module id="signin-dialog-shared">
   <template>
     <style>
       a {
         color: var(--google-blue-700);
         text-decoration: none;
       }
 
       body {
         margin: 0;
+        /* Scrollbars are hidden as the sign-in dialogs are automatically
+        resized.*/
+        overflow: hidden;
         padding: 0;
       }
 
       paper-button {
         font-weight: 500;
         line-height: 16px;
         margin: 0;
         padding: 8px 16px;
       }
 
       paper-button.primary-action {
         --paper-button-flat-keyboard-focus: {
           background: rgb(58, 117, 215);
         };
         background: var(--google-blue-500);
         color: white;
       }
 
       paper-button.secondary-action {
         --paper-button-flat-keyboard-focus: {
           background: rgba(0, 0, 0, .12);
         };
         color: var(--paper-grey-600);
       }
 
       .container {
         background-color: white;
         color: #333;
+        /* Scrollbars are hidden as the sign-in dialogs are automatically
+        resized.*/
         overflow: hidden;
         width: 448px;
       }
 
       .top-title-bar {
         align-items: center;
         border-bottom: 1px solid var(--paper-grey-300);
         display: flex;
         font-size: 16px;
         height: 52px;
         padding: 0 24px;
       }
 
       .action-container {
         display: flex;
         justify-content: flex-end;
         padding: 16px;
       }
 
 <if expr="is_macosx or is_linux">
       .action-container {
         flex-flow: row-reverse;
         justify-content: flex-start;
       }
 </if>
 
     </style>
   </template>
 </dom-module>