Add file access modes to sk_exists.

src/ports/SkOSFile_win.cpp

Index: src/ports/SkOSFile_win.cpp
diff --git a/src/ports/SkOSFile_win.cpp b/src/ports/SkOSFile_win.cpp
index a0848915ba49e9fd6f8f34b25ba70ca3c003f41a..1430d7f03bf3d1147c2dd4e0be3cd5edfeba4dc5 100644
--- a/src/ports/SkOSFile_win.cpp
+++ b/src/ports/SkOSFile_win.cpp
@@ -13,6 +13,17 @@
 #include <stdio.h>
 #include <sys/stat.h>
 
+bool sk_exists(const char *path, SkFILE_Flags flags) {
+    int mode = 0; // existence
+    if (flags & kRead_SkFILE_Flag) {
+        mode |= 4; // read
+    }
+    if (flags & kWrite_SkFILE_Flag) {
+        mode |= 2; // write
+    }
+    return (0 == _access(path, mode));
+}
+
 typedef struct {
     ULONGLONG fVolume;
     ULONGLONG fLsbSize;