Make a pen in eraser mode visible thru PointerEvent.buttons

third_party/WebKit/Source/core/events/MouseEvent.h

Index: third_party/WebKit/Source/core/events/MouseEvent.h
diff --git a/third_party/WebKit/Source/core/events/MouseEvent.h b/third_party/WebKit/Source/core/events/MouseEvent.h
index 36bf6385fa21cdab413b89b179f106d7e7f223f1..15ed5dcb22fc97795837f100638a4ce1a28139ab 100644
--- a/third_party/WebKit/Source/core/events/MouseEvent.h
+++ b/third_party/WebKit/Source/core/events/MouseEvent.h
@@ -92,11 +92,18 @@ public:
 
     EventDispatchMediator* createMediator() override;
 
+    // Follows the definition of |buttons| in the PointerEvent Spec:
+    //   https://w3c.github.io/pointerevents/#the-buttons-property
+    // which extends the UI Events Spec definition:
+    //   https://www.w3.org/TR/uievents/#interface-mouseevent
     enum class Buttons : unsigned {
         None = 0,
         Left = 1 << 0,
         Right = 1 << 1,
-        Middle = 1 << 2
+        Middle = 1 << 2,
+        X1 = 1 << 3,
+        X2 = 1 << 4,
+        Eraser = 1 << 5
     };
 
     DECLARE_VIRTUAL_TRACE();