Limit the length of managed user name to 50 symbols.

chrome/browser/resources/chromeos/login/screen_locally_managed_user_creation.html

 <div class="step hidden" id="managed-user-creation">
   <div class="step-contents">
     <div id="managed-user-creation-intro" class="step-no-logo">
       <div id="managed-user-creation-marketing-intro"
           class="marketing">
         <img src="chrome://theme/IDR_SUPERVISED_ILLUSTRATION_START">
       </div>
       <div class="below-marketing">
         <div class="page-title" i18n-content="createManagedUserIntroTextTitle">
         </div>
@@ skipping 16 matching lines @@
     </div>
     <div id="managed-user-creation-username" class="page-no-marketing" hidden>
       <div class="logo-padded-text">
         <div>
           <div class="page-title inline"
               i18n-content="createManagedUserNameTitle"></div>
           <div id="managed-user-creation-name-explanation"
               class="page-title-explanation inline"></div>
         </div>
         <div id="managed-user-creation-name-block">
-          <input id="managed-user-creation-name" type="text" />
+          <input id="managed-user-creation-name" type="text" maxlength="50" />
           <span id="managed-user-creation-name-error" class="no-error">
           </span>
         </div>
         <div>
           <div class="page-title inline"
               i18n-content="createManagedUserPasswordTitle"></div>
           <div class="page-title-explanation inline"
               i18n-content="createManagedUserPasswordExplanation"></div>
         </div>
         <div id="managed-user-creation-password-block">
@@ skipping 75 matching lines @@
         <div class="managed-user-creation-manager-name"></div>
         <div class="managed-user-creation-manager-email"></div>
       </div>
     </div>
     <div class="password-block" hidden>
       <input class="managed-user-creation-manager-password" type="password"
           i18n-values="placeholder:createManagedUserManagerPasswordHint" />
     </div>
   </div>
 </div>