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

chrome/test/automation/proxy_launcher.cc

Index: chrome/test/automation/proxy_launcher.cc
diff --git a/chrome/test/automation/proxy_launcher.cc b/chrome/test/automation/proxy_launcher.cc
index 2fb04c6c2595dbccb78e2b9cd64474fb6e9f3c89..04915383808b0445e45a615d9f6d0408a145d0dc 100644
--- a/chrome/test/automation/proxy_launcher.cc
+++ b/chrome/test/automation/proxy_launcher.cc
@@ -499,11 +499,11 @@ base::TimeDelta ProxyLauncher::browser_quit_time() const {
 // NamedProxyLauncher functions
 
 NamedProxyLauncher::NamedProxyLauncher(const std::string& channel_id,
-                                       bool launch_browser,
-                                       bool disconnect_on_failure)
+                                       const InitParams& params)
     : channel_id_(channel_id),
-      launch_browser_(launch_browser),
-      disconnect_on_failure_(disconnect_on_failure) {
+      launch_browser_(params.launch_browser),
+      disconnect_on_failure_(params.disconnect_on_failure),
+      wait_for_server_channel_(params.wait_for_server_channel) {
 }
 
 AutomationProxy* NamedProxyLauncher::CreateAutomationProxy(
@@ -514,7 +514,7 @@ AutomationProxy* NamedProxyLauncher::CreateAutomationProxy(
   return proxy;
 }
 
-void NamedProxyLauncher::InitializeConnection(const LaunchState& state,
+bool NamedProxyLauncher::InitializeConnection(const LaunchState& state,
                                               bool wait_for_initial_loads) {
   FilePath testing_channel_path;
 #if defined(OS_WIN)
@@ -526,25 +526,40 @@ void NamedProxyLauncher::InitializeConnection(const LaunchState& state,
   if (launch_browser_) {
     // Because we are waiting on the existence of the testing file below,
     // make sure there isn't one already there before browser launch.
-    EXPECT_TRUE(file_util::Delete(testing_channel_path, false));
+    if (!file_util::Delete(testing_channel_path, false)) {
+      LOG(ERROR) << "Failed to delete " << testing_channel_path.value();
+      return false;
+    }
+
+    if (!LaunchBrowser(state)) {
+      LOG(ERROR) << "Failed to LaunchBrowser";
+      return false;
+    }
+  }
 
-    // Set up IPC testing interface as a client.
-    ASSERT_TRUE(LaunchBrowser(state));
+  if (wait_for_server_channel_) {
+    // Wait for browser to be ready for connections.
+    bool testing_channel_exists = false;
+    for (int wait_time = 0;
+         wait_time < TestTimeouts::action_max_timeout_ms();
+         wait_time += automation::kSleepTime) {
+      testing_channel_exists = file_util::PathExists(testing_channel_path);
+      if (testing_channel_exists)
+        break;
+      base::PlatformThread::Sleep(automation::kSleepTime);
+    }
+    if (!testing_channel_exists) {
+      LOG(ERROR) << "Failed to wait for testing channel presence.";
+      return false;
+    }
   }
 
-  // Wait for browser to be ready for connections.
-  bool testing_channel_exists = false;
-  for (int wait_time = 0;
-       wait_time < TestTimeouts::action_max_timeout_ms();
-       wait_time += automation::kSleepTime) {
-    testing_channel_exists = file_util::PathExists(testing_channel_path);
-    if (testing_channel_exists)
-      break;
-    base::PlatformThread::Sleep(automation::kSleepTime);
+  if (!ConnectToRunningBrowser(wait_for_initial_loads)) {
+    LOG(ERROR) << "Failed to ConnectToRunningBrowser";
+    return false;
   }
-  EXPECT_TRUE(testing_channel_exists);
 
-  ASSERT_TRUE(ConnectToRunningBrowser(wait_for_initial_loads));
+  return true;
 }
 
 void NamedProxyLauncher::TerminateConnection() {
@@ -575,9 +590,9 @@ AutomationProxy* AnonymousProxyLauncher::CreateAutomationProxy(
   return proxy;
 }
 
-void AnonymousProxyLauncher::InitializeConnection(const LaunchState& state,
+bool AnonymousProxyLauncher::InitializeConnection(const LaunchState& state,
                                                   bool wait_for_initial_loads) {
-  ASSERT_TRUE(LaunchBrowserAndServer(state, wait_for_initial_loads));
+  return LaunchBrowserAndServer(state, wait_for_initial_loads);
 }
 
 void AnonymousProxyLauncher::TerminateConnection() {