[ChromeDriver] Add support for attaching multiple DevToolsEventListeners to a DevToolsClient.

chrome/test/chromedriver/chrome_impl_unittest.cc

Index: chrome/test/chromedriver/chrome_impl_unittest.cc
diff --git a/chrome/test/chromedriver/chrome_impl_unittest.cc b/chrome/test/chromedriver/chrome_impl_unittest.cc
index 56bb884a1997dc4cdf3a40aa3b4fe0c77b836218..8b52ad2c78e27d063bb1285d47887db5cfacba3b 100644
--- a/chrome/test/chromedriver/chrome_impl_unittest.cc
+++ b/chrome/test/chromedriver/chrome_impl_unittest.cc
@@ -95,6 +95,7 @@ class FakeDevToolsClient : public DevToolsClient {
     result->reset(result_.DeepCopy());
     return Status(kOk);
   }
+  virtual void AddListener(DevToolsEventListener* listener) OVERRIDE {}
 
  private:
   Status status_;