Remove enable-iframe-based-signin flag.

chrome/app/google_chrome_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 <!-- This file contains definitions of strings that are specific to
 Google Chrome. These strings will be translated for each locale that Google
 Chrome supports. If you update this file, be sure also to update
 chromium_strings.grd. -->
 
 <grit base_dir="." latest_public_release="0" current_release="1"
       source_lang_id="en" enc_check="möl">
   <outputs>
     <output filename="grit/google_chrome_strings.h" type="rc_header">
@@ skipping 535 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 Chrome was automatically launched on startup">
           Google Chrome 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 Chrome 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 Chrome 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 Google Chrome.
       </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.">
          Google Chrome 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 Chrome apps.
         </message>
       </if>
 
     </messages>
   </release>
 </grit>