Sync UI: Add a flag to enable the new signin flow.

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
diff --git a/chrome/app/generated_resources.grd b/chrome/app/generated_resources.grd
index 682a74fe92ea0d80e29b50dfdac985d3d0ffe87c..736a48275fc81cd85c4002cd91aa9ae0fa64749c 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -11464,6 +11464,14 @@ Some features may be unavailable.  Please check that the profile exists and you
         Sync service is not available for your domain.
       </message>
 
+      <!-- Sync Signin Flag -->
+      <message name="IDS_ENABLE_SYNC_SIGNIN_NAME" desc="Title of the enable sync signin flag." >
+        Enable sync signin
+      </message>
+      <message name="IDS_ENABLE_SYNC_SIGNIN_DESCRIPTION" desc="Description of the enable sync signin flag.">
+        Enables a new sync signin flow where all signin UI is forwarded to chrome:signin.
+      </message>
+
       <!-- Web Store login dialog strings -->
       <message name="IDS_WEB_STORE_LOGIN_INTRODUCTION_1" desc="The first message to display in the Web Store login dialog.">
         To use the Chrome Web Store, you must sign in with a Google Account.