Add the IDL and stub implementation for the chrome.autofillPrivate API.

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 470a5b69488eb7b30409bd9941c36b8135a16aea..96a7b5d06e986b91f7c355b82fb8320d9c51bb45 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -4603,6 +4603,9 @@ Even if you have downloaded files from this website before, the website might ha
       <message name="IDS_EXTENSION_PROMPT_WARNING_SEARCH_ENGINES_PRIVATE" desc="Permission string for access to search engines.">
         Read and change default and custom search engines
       </message>
+      <message name="IDS_EXTENSION_PROMPT_WARNING_AUTOFILL_PRIVATE" desc="Permission string for access to autofill settings.">
+        Read and change autofill settings
+      </message>
 
       <!-- Extension/App error messages -->
       <message name="IDS_EXTENSION_CANT_GET_ABSOLUTE_PATH" desc="Warning displayed in pack dialog when the absolute path to the extension directory can not be found.">