[Extensions UI] Add a placeholder icon for extensions in the toolbar

chrome/browser/extensions/extension_action.h

Index: chrome/browser/extensions/extension_action.h
diff --git a/chrome/browser/extensions/extension_action.h b/chrome/browser/extensions/extension_action.h
index ed20da29bd5aafaf1a5e769d031111475fcc79f6..efdceeef6509828d233fbc761dd4228563962c0b 100644
--- a/chrome/browser/extensions/extension_action.h
+++ b/chrome/browser/extensions/extension_action.h
@@ -257,6 +257,9 @@ class ExtensionAction {
   // extension manifest).
   const std::string extension_id_;
 
+  // The name of the extension.
+  const std::string extension_name_;
+
   const extensions::ActionInfo::Type action_type_;
 
   // Each of these data items can have both a global state (stored with the key