Partially deprecate the old webstorePrivate.beginInstall method

chrome/browser/extensions/extension_install_ui.h

Index: chrome/browser/extensions/extension_install_ui.h
diff --git a/chrome/browser/extensions/extension_install_ui.h b/chrome/browser/extensions/extension_install_ui.h
index 44e2f2617f80af0e494684ec6d9fcaaf7e4a1c21..334815807c86735d4586fe7cd21bed22e8ea2bc4 100644
--- a/chrome/browser/extensions/extension_install_ui.h
+++ b/chrome/browser/extensions/extension_install_ui.h
@@ -82,6 +82,8 @@ class ExtensionInstallUI : public ImageLoadingTracker::Observer {
   static void DisableFailureUIForTests();
 
  private:
+  friend class GalleryInstallApiTestObserver;
+
   // Show an infobar for a newly-installed theme.  previous_theme_id
   // should be empty if the previous theme was the system/default
   // theme.