Remove enable-iframe-based-signin flag.

chrome/app/chromium_strings.grd

Index: chrome/app/chromium_strings.grd
diff --git a/chrome/app/chromium_strings.grd b/chrome/app/chromium_strings.grd
index 4e2cbfeab115a77e9df6df07fe1e1a4e6faf6a4a..96dd97e559bb0f34966c0a5ba007f9a410bc2d1d 100644
--- a/chrome/app/chromium_strings.grd
+++ b/chrome/app/chromium_strings.grd
@@ -551,12 +551,6 @@ Chromium is unable to recover your settings.
       </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>