Change Skia-related proto enums to uppercase

cc/proto/skregion.proto

 // Copyright 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.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package cc.proto;
 
 message SkRegion {
   enum Op {
-    Op_Difference = 0;
-    Op_Intersect = 1;
-    Op_Union = 2;
-    Op_XOR = 3;
-    Op_ReverseDifference = 4;
-    Op_Replace = 5;
+    DIFFERENCE_ = 0;  // On Windows, winuser.h defines DIFFERENCE.
+    INTERSECT = 1;
+    UNION = 2;
+    XOR = 3;
+    REVERSE_DIFFERENCE = 4;
+    REPLACE = 5;
   }
-}
+}