Add file access modes to sk_exists.

src/ports/SkOSFile_none.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkOSFile.h"
 
+bool sk_exists(const char *path, SkFILE_Flags flags) {
+    return false;
+}
+
 bool sk_fidentical(SkFILE* a, SkFILE* b) {
     return false;
 }
 
 int sk_fileno(SkFILE* f) {
     return -1;
 }
 
 void sk_fmunmap(const void* addr, size_t length) { }
 
 void* sk_fdmmap(int fd, size_t* size) {
     return NULL;
 }
 
 void* sk_fmmap(SkFILE* f, size_t* size) {
     return NULL;
 }