New post-sideload UI:

chrome/browser/extensions/extension_install_prompt.h

Index: chrome/browser/extensions/extension_install_prompt.h
diff --git a/chrome/browser/extensions/extension_install_prompt.h b/chrome/browser/extensions/extension_install_prompt.h
index ce32fef9826e9aed7e45aa47cb70af633a180979..3354fb74c739790250574c1589a7be555fd0b4b3 100644
--- a/chrome/browser/extensions/extension_install_prompt.h
+++ b/chrome/browser/extensions/extension_install_prompt.h
@@ -53,6 +53,7 @@ class ExtensionInstallPrompt : public ImageLoadingTracker::Observer,
     BUNDLE_INSTALL_PROMPT,
     RE_ENABLE_PROMPT,
     PERMISSIONS_PROMPT,
+    EXTERNAL_INSTALL_PROMPT,
     NUM_PROMPT_TYPES
   };
 
@@ -233,6 +234,13 @@ class ExtensionInstallPrompt : public ImageLoadingTracker::Observer,
   virtual void ConfirmReEnable(Delegate* delegate,
                                const extensions::Extension* extension);
 
+  // This is called by the external install alert UI to verify whether the
+  // extension should be enabled (external extensions are installed disabled).
+  //
+  // We *MUST* eventually call either Proceed() or Abort() on |delegate|.
+  virtual void ConfirmExternalInstall(Delegate* delegate,
+                                      const extensions::Extension* extension);
+
   // This is called by the extension permissions API to verify whether an
   // extension may be granted additional permissions.
   //