Ignore left-button eventSender.mouseDown() uses while dragging.

content/shell/renderer/test_runner/event_sender.cc

Index: content/shell/renderer/test_runner/event_sender.cc
diff --git a/content/shell/renderer/test_runner/event_sender.cc b/content/shell/renderer/test_runner/event_sender.cc
index b22fe7291c6bcc4be1a8a99aaeffa86cc188e650..fa8685ba5776a051ec4a6fe8a3020276374aa0f5 100644
--- a/content/shell/renderer/test_runner/event_sender.cc
+++ b/content/shell/renderer/test_runner/event_sender.cc
@@ -1189,6 +1189,11 @@ void EventSender::MouseDown(int button_number, int modifiers) {
   WebMouseEvent::Button button_type =
       GetButtonTypeFromButtonNumber(button_number);
 
+  // Ignore invalid uses of mouseDown() while drag is in progress.
+  if (button_type == WebMouseEvent::ButtonLeft &&
+      !current_drag_data_.isNull())
+    return;
+
   UpdateClickCountForButton(button_type);
 
   pressed_button_ = button_type;