[Panels] Provide a tab id for web contents in Panels.

chrome/browser/extensions/extension_messages_apitest.cc

Index: chrome/browser/extensions/extension_messages_apitest.cc
diff --git a/chrome/browser/extensions/extension_messages_apitest.cc b/chrome/browser/extensions/extension_messages_apitest.cc
index 9a0373039db382494bd042982c3d8a7b2de0276d..2a47ba66f45d85e715efa3d7cf9fa382683a4c2e 100644
--- a/chrome/browser/extensions/extension_messages_apitest.cc
+++ b/chrome/browser/extensions/extension_messages_apitest.cc
@@ -90,3 +90,8 @@ IN_PROC_BROWSER_TEST_F(ExtensionApiTest, MessagingEventURL) {
   MessageSender sender;
   ASSERT_TRUE(RunExtensionTest("messaging/event_url")) << message_;
 }
+
+// Tests connecting from a panel to its extension.
+IN_PROC_BROWSER_TEST_F(ExtensionApiTest, MessagingPanel) {
+  ASSERT_TRUE(RunExtensionTest("messaging/connect_panel")) << message_;
+}