Don't crash on chrome.runtime.connect().

extensions/renderer/resources/runtime_custom_bindings.js

Index: extensions/renderer/resources/runtime_custom_bindings.js
diff --git a/extensions/renderer/resources/runtime_custom_bindings.js b/extensions/renderer/resources/runtime_custom_bindings.js
index 60c3bd02e2a0106153c3e810397fe3fe592b7d7d..4e4a71d1e0a2c7b313c4d29d7123ac6b68bbc3ce 100644
--- a/extensions/renderer/resources/runtime_custom_bindings.js
+++ b/extensions/renderer/resources/runtime_custom_bindings.js
@@ -158,8 +158,16 @@ binding.registerCustomHook(function(binding, id, contextType) {
     if (unloadEvent.wasDispatched)
       throw new Error('Error connecting to extension ' + targetId);
 
-    if (!targetId)
+    if (!targetId) {
+      // runtime.id is only defined inside extensions. If we're in a webpage,
+      // the best we can do at this point is to fail.
+      if (!runtime.id) {
+        throw new Error('chrome.runtime.connect() called from a webpage must ' +
+                        'specify an Extension ID (string) for its first ' +
+                        'argument');
+      }
       targetId = runtime.id;
+    }
 
     var name = '';
     if (connectInfo && connectInfo.name)