Add file access modes to sk_exists.

src/ports/SkOSFile_posix.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"
 
 #include "SkTFitsIn.h"
 
 #include <stdio.h>
 #include <sys/mman.h>
 #include <sys/stat.h>
 #include <sys/types.h>
+#include <unistd.h>
+
+bool sk_exists(const char *path, SkFILE_Flags flags) {
+    int mode = F_OK;
+    if (flags & kRead_SkFILE_Flag) {
+        mode |= R_OK;
+    }
+    if (flags & kWrite_SkFILE_Flag) {
+        mode |= W_OK;
+    }
+    return (0 == access(path, mode));
+}
 
 typedef struct {
     dev_t dev;
     ino_t ino;
 } SkFILEID;
 
 static bool sk_ino(SkFILE* a, SkFILEID* id) {
     int fd = fileno((FILE*)a);
     if (fd < 0) {
         return 0;
@@ skipping 45 matching lines @@
 }
 
 void* sk_fmmap(SkFILE* f, size_t* size) {
     int fd = sk_fileno(f);
     if (fd < 0) {
         return NULL;
     }
 
     return sk_fdmmap(fd, size);
 }