fix skpdiff viewer bug when using relative paths

tools/skpdiff/skpdiff_util.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef skpdiff_util_DEFINED
 #define skpdiff_util_DEFINED
 
@@ skipping 31 matching lines @@
 bool get_directory(const char path[], SkTArray<SkString>* entries);
 
 /**
  * Gets the files that match the specified pattern in sorted order.
  * @param  globPattern The pattern to use. Patterns must be valid paths, optionally with wildcards (*)
  * @param  entries     An array to return the results into
  * @return             True on success, false otherwise
  */
 bool glob_files(const char globPattern[], SkTArray<SkString>* entries);
 
+/**
+ * Gets the absolute version of the given path.
+ * @param  path The absolute or relative path to expand
+ * @return      The absolute path of the given path on success, or an empty string on failure.
+ */
+SkString get_absolute_path(const SkString& path);
+
 
 #endif