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

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

+var pass = chrome.test.callbackPass;
+var fail = chrome.test.callbackFail;
+
+var debuggee;
+var protocolVersion = "1.0";
+
+chrome.test.runTests([
+
+  function attach() {
+    var extensionId = chrome.extension.getURL('').split('/')[2];
+    debuggee = {extensionId: extensionId};
+    chrome.debugger.attach(debuggee, protocolVersion, pass());
+  },
+
+  function attachToMissing() {
+    var missingDebuggee = {extensionId: "foo"};
+    chrome.debugger.attach(missingDebuggee, protocolVersion,
+        fail("No extension with given id " +
+            missingDebuggee.extensionId + "."));
+  },
+
+  function attachAgain() {
+    chrome.debugger.attach(debuggee, protocolVersion,
+        fail("Another debugger is already attached to the extension with id: " +
+            debuggee.extensionId + "."));
+  },
+
+  function detach() {
+    chrome.debugger.detach(debuggee, pass());
+  },
+
+  function detachAgain() {
+    chrome.debugger.detach(debuggee,
+        fail("Debugger is not attached to the extension with id: " +
+            debuggee.extensionId + "."));
+  },
+]);