Adds a webstorePrivate method for silently installing extensions.

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index ca91aeea7d594a77a2222dca58af6f9716248b47..52dfbf4574559952f54f3f3f644cf3327f75f4b9 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -75,6 +75,10 @@ const char kAppsGalleryReturnTokens[]       = "apps-gallery-return-tokens";
 // The URL to use for the gallery link in the app launcher.
 const char kAppsGalleryURL[]                = "apps-gallery-url";
 
+// The URL that the webstore APIs download extensions from.
+// Note: the URL must contain one '%s' for the extension ID.
+const char kAppsGalleryDownloadURL[]        = "apps-gallery-download-url";
+
 // The update url used by gallery/webstore extensions.
 const char kAppsGalleryUpdateURL[]          = "apps-gallery-update-url";