Limit extensions to 1 UI surface (either Page/Browser action, but not both)....

chrome/common/extensions/extension.cc

Index: chrome/common/extensions/extension.cc
===================================================================
--- chrome/common/extensions/extension.cc	(revision 29153)
+++ chrome/common/extensions/extension.cc	(working copy)
@@ -958,6 +958,12 @@
 
   // Initialize browser action (optional).
   if (source.HasKey(keys::kBrowserAction)) {
+    // Restrict extensions to one UI surface.
+    if (source.HasKey(keys::kPageAction) || source.HasKey(keys::kPageActions)) {
+      *error = errors::kOneUISurfaceOnly;
+      return false;
+    }
+
     DictionaryValue* browser_action_value;
     if (!source.GetDictionary(keys::kBrowserAction, &browser_action_value)) {
       *error = errors::kInvalidBrowserAction;