Bug 1000870 - Add some features in testing system. r=smaug

dom/base/nsContentUtils.cpp

@@ -7669,6 +7669,8 @@ int16_t
 nsContentUtils::GetButtonsFlagForButton(int32_t aButton)
 {
   switch (aButton) {
+    case -1:
+      return WidgetMouseEvent::eNoButtonFlag;
     case WidgetMouseEvent::eLeftButton:
       return WidgetMouseEvent::eLeftButtonFlag;
     case WidgetMouseEvent::eMiddleButton:

widget/MouseEvents.h

@@ -115,6 +115,7 @@ public:
   int16_t button;
 
   enum buttonsFlag {
+    eNoButtonFlag     = 0x00,
     eLeftButtonFlag   = 0x01,
     eRightButtonFlag  = 0x02,
     eMiddleButtonFlag = 0x04,