Use SkToBool to fix a warning.

Review URL: https://codereview.appspot.com/6818069

git-svn-id: http://skia.googlecode.com/svn/trunk@6240 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/ports/SkOSFile_stdio.cpp b/src/ports/SkOSFile_stdio.cpp
index 6c825b4..9100f6d 100644
--- a/src/ports/SkOSFile_stdio.cpp
+++ b/src/ports/SkOSFile_stdio.cpp
@@ -122,7 +122,7 @@
     if (0 != stat(path, &status)) {
         return false;
     }
-    return (status.st_mode & S_IFDIR);
+    return SkToBool(status.st_mode & S_IFDIR);
 }
 
 bool sk_mkdir(const char* path)