Let pyauto create an attached webdriver instance to manipulate web pages.

chrome/test/automation/proxy_launcher.h

Index: chrome/test/automation/proxy_launcher.h
diff --git a/chrome/test/automation/proxy_launcher.h b/chrome/test/automation/proxy_launcher.h
index d306a334fd87ba5fbbb3fbd77de186dcb789df1f..e2a273a164494cd714ee2bb915102d427e1e3b39 100644
--- a/chrome/test/automation/proxy_launcher.h
+++ b/chrome/test/automation/proxy_launcher.h
@@ -65,8 +65,10 @@ class ProxyLauncher {
   virtual ~ProxyLauncher();
 
   // Launches the browser if needed and establishes a connection with it.
-  virtual void InitializeConnection(const LaunchState& state,
-                                    bool wait_for_initial_loads) = 0;
+  // Returns true on success.
+  virtual bool InitializeConnection(
+      const LaunchState& state,
+      bool wait_for_initial_loads) WARN_UNUSED_RESULT = 0;
 
   // Shuts down the browser if needed and destroys any
   // connections established by InitalizeConnection.
@@ -282,21 +284,35 @@ class ProxyLauncher {
 // The channel id of the proxy is a constant specified by kInterfacePath.
 class NamedProxyLauncher : public ProxyLauncher {
  public:
-  // If launch_browser is true, launches Chrome with named interface enabled.
-  // Otherwise, there should be an existing instance the proxy can connect to.
+  struct InitParams {
+    // True if the browser should be launched. If false, this will connect
+    // to an already started Chrome.
+    bool launch_browser;
+
+    // True if we should disconnect on an IPC failure.
+    bool disconnect_on_failure;
+
+    // True if we should wait for Chrome's server channel to be initialized
+    // and listening. Only set to false if it is guaranteed the server channel
+    // is already listening.
+    bool wait_for_server_channel;
+  };
+
   NamedProxyLauncher(const std::string& channel_id,
-                     bool launch_browser, bool disconnect_on_failure);
+                     const InitParams& params);
 
   virtual AutomationProxy* CreateAutomationProxy(int execution_timeout);
-  virtual void InitializeConnection(const LaunchState& state,
-                                    bool wait_for_initial_loads);
+  virtual bool InitializeConnection(
+      const LaunchState& state,
+      bool wait_for_initial_loads) OVERRIDE WARN_UNUSED_RESULT;
   virtual void TerminateConnection();
   virtual std::string PrefixedChannelID() const;
 
  protected:
-  std::string channel_id_;      // Channel id of automation proxy.
-  bool launch_browser_;         // True if we should launch the browser too.
-  bool disconnect_on_failure_;  // True if we disconnect on IPC channel failure.
+  std::string channel_id_;        // Channel id of automation proxy.
+  bool launch_browser_;           // True if we should launch the browser too.
+  bool disconnect_on_failure_;    // True if we disconnect on channel failure.
+  bool wait_for_server_channel_;  // True if we should wait for server channel.
 
  private:
   DISALLOW_COPY_AND_ASSIGN(NamedProxyLauncher);
@@ -307,8 +323,9 @@ class AnonymousProxyLauncher : public ProxyLauncher {
  public:
   explicit AnonymousProxyLauncher(bool disconnect_on_failure);
   virtual AutomationProxy* CreateAutomationProxy(int execution_timeout);
-  virtual void InitializeConnection(const LaunchState& state,
-                                    bool wait_for_initial_loads);
+  virtual bool InitializeConnection(
+      const LaunchState& state,
+      bool wait_for_initial_loads) OVERRIDE WARN_UNUSED_RESULT;
   virtual void TerminateConnection();
   virtual std::string PrefixedChannelID() const;