[Autofill] Implement Impression metric for signin promo in CC dropdown.

tools/metrics/actions/actions.xml

 <!--
 Copyright 2014 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <!--
 This file is a comprehensive list of Chrome user actions along with the owner
 and description for each user action. A user action should be tied to an actual
 action taken by a user. It should be logged immediately after a user takes the
@@ skipping 13286 matching lines @@
   </description>
 </action>
 
 <action name="Signin_Impression_FromAppsPageLink">
   <owner>gogerald@chromium.org</owner>
   <description>
     Recorded when showing sign in entry in the chrome://apps page.
   </description>
 </action>
 
+<action name="Signin_Impression_FromAutofillDropdown">
+  <owner>mathp@chromium.org</owner>
+  <description>
+    Recorded when showing sign in entry in the Autofill dropdown.
+  </description>
+</action>
+
 <action name="Signin_Impression_FromAvatarBubbleSignin">
   <owner>gogerald@chromium.org</owner>
   <description>
     Recorded when showing sign in entry in the avatar sign in bubble.
   </description>
 </action>
 
 <action name="Signin_Impression_FromBookmarkBubble">
   <owner>gogerald@chromium.org</owner>
   <description>
@@ skipping 1729 matching lines @@
 <action name="webapps.AddShortcut.Manifest">
   <owner>mlamouri@chromium.org</owner>
   <description>
     Shortcut added to the homescreen with a valid Manifest. This action will be
     recorded in addition to AppShortcut, AppShortcutApple and Bookmark actions
     to keep a trend of whether the webapp also add meta tags.
   </description>
 </action>
 
 </actions>