Allow chrome.debugger API to attach to extension background pages.

chrome/test/data/extensions/api_test/debugger/background.js

Index: chrome/test/data/extensions/api_test/debugger/background.js
diff --git a/chrome/test/data/extensions/api_test/debugger/background.js b/chrome/test/data/extensions/api_test/debugger/background.js
index b6b65f5c6ddffe5a214d1d84e96574768a26ba7f..afa7678779328f36721316cdc9b4df4517279ec4 100644
--- a/chrome/test/data/extensions/api_test/debugger/background.js
+++ b/chrome/test/data/extensions/api_test/debugger/background.js
@@ -98,6 +98,18 @@ chrome.test.runTests([
           fail("Can not attach to the page with the \"chrome://\" scheme."));
       chrome.tabs.remove(tab.id);
     });
-  }
+  },
 
+  function attachToMissing() {
+    var missingDebuggee = {tabId: -1};
+    chrome.debugger.attach(missingDebuggee, protocolVersion,
+        fail("No tab with given id " + missingDebuggee.tabId + "."));
+  },
+
+  function attachToExtensionWithNoSilentFlag() {
+    debuggeeExtension = {extensionId: "foo"};
+    chrome.debugger.attach(debuggeeExtension, protocolVersion,
+        fail("Cannot attach to an extension, " +
+            "--enable-silent-debugging command line option required."));
+  }
 ]);