Add validation to a few WebPointerProperties.

third_party/WebKit/public/platform/WebPointerProperties.h

 // Copyright (c) 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef WebPointerProperties_h
 #define WebPointerProperties_h
 
 #include <cstdint>
 #include <limits>
 
 namespace blink {
 
 using PointerId = int32_t;
 
 // This class encapsulates the properties that are common between mouse and
 // pointer events and touch points as we transition towards the unified pointer
 // event model.
 // TODO(e_hakkinen): Replace WebTouchEvent with WebPointerEvent, remove
 // WebTouchEvent and WebTouchPoint and merge this into WebPointerEvent.
 class WebPointerProperties {
  public:
   enum class Button {
     kNoButton = -1,
     kLeft,
     kMiddle,
     kRight,
     kBack,
     kForward,
-    kEraser
+    kEraser,
+    kLastEntry = kEraser  // Must be the last entry in the list
   };
 
   enum class Buttons : unsigned {
     kNoButton = 0,
     kLeft = 1 << 0,
     kRight = 1 << 1,
     kMiddle = 1 << 2,
     kBack = 1 << 3,
     kForward = 1 << 4,
     kEraser = 1 << 5
@@ skipping 65 matching lines @@
 
   PointerType pointer_type;
 
   int movement_x;
   int movement_y;
 };
 
 }  // namespace blink
 
 #endif