Remove enable-iframe-based-signin flag.

chrome/app/chromium_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 <!-- This file contains definitions of strings that are distribution specific.
 If you update this file, be sure also to update google_chrome_strings.grd. -->
 
 <grit base_dir="." latest_public_release="0" current_release="1"
       source_lang_id="en" enc_check="möl">
   <outputs>
     <output filename="grit/chromium_strings.h" type="rc_header">
       <emit emit_type='prepend'></emit>
     </output>
@@ skipping 533 matching lines @@
       </if>
 
       <!-- Autolaunch infobar -->
       <if expr="is_win">
         <message name="IDS_AUTO_LAUNCH_INFOBAR_TEXT" desc="The text to show in the infobar when Chromium was automatically launched on startup">
           Chromium is configured to automatically launch when you start your computer.
         </message>
       </if>
 
       <!-- about:flags -->
-      <message name="IDS_FLAGS_ENABLE_IFRAME_BASED_SIGNIN_NAME" desc="Title for the flag to enable iframe-based sign-in flows.">
-        Enables iframe-based Chromium sign-in flows. This flag overrides --enable-web-based-signin.
-      </message>
-      <message name="IDS_FLAGS_ENABLE_IFRAME_BASED_SIGNIN_DESCRIPTION" desc="Description for the flag to enable iframe-based sign-in flows">
-        When enabled, will use an iframe-based Chromium sign-in flow; otherwise uses a webview-based flow.
-      </message>
       <message name="IDS_FLAGS_DEBUG_SHORTCUTS_DESCRIPTION" desc="Description of the 'Debugging keyboard shortcuts' lab.">
         Enables additional keyboard shortcuts that are useful for debugging Chromium.
       </message>
 
       <!-- Obsolete System info bar -->
       <message name="IDS_SYSTEM_OBSOLETE_MESSAGE" desc="Message shown when your OS is no longer supported. This message is followed by a 'Learn more' link.">
          Chromium has stopped updating and no longer supports this version of your operating system.
       </message>
 
       <!-- Google API keys info bar -->
@@ skipping 713 matching lines @@
         </message>
         <message name="IDS_WIN_PROMPT_TO_CLOSE_APPS_FOR_DESKTOP"
                  desc="Modal message box prompt shown to the user after requesting to relaunch in Desktop mode while packaged apps are running, which will require all packaged apps to be closed and then restarted. Dialog is shown with choices: OK, Cancel.">
           Relaunching in Desktop mode will close and relaunch your Chromium apps.
         </message>
       </if>
 
     </messages>
   </release>
 </grit>