fix skpdiff viewer bug when using relative paths

tools/skpdiff/skpdiff_util.h

Index: tools/skpdiff/skpdiff_util.h
diff --git a/tools/skpdiff/skpdiff_util.h b/tools/skpdiff/skpdiff_util.h
index 9df1bc0b2a157f982dc49112d73fccee11a54f04..8750bf6b54d15a07798cb9c3248921ada46d5611 100644
--- a/tools/skpdiff/skpdiff_util.h
+++ b/tools/skpdiff/skpdiff_util.h
@@ -49,5 +49,12 @@ bool get_directory(const char path[], SkTArray<SkString>* entries);
  */
 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