Treat bad values passed to --images as a fatal error

tools/flags/SkCommonFlags.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SK_COMMON_FLAGS_H
 #define SK_COMMON_FLAGS_H
 
 #include "SkCommandLineFlags.h"
+#include "SkTArray.h"
+#include "SkString.h"
 
 DECLARE_bool(cpu);
 DECLARE_bool(dryRun);
 DECLARE_bool(gpu);
 DECLARE_string(images);
 DECLARE_string(match);
 DECLARE_bool(quiet);
 DECLARE_bool(resetGpuContext);
 DECLARE_bool(preAbandonGpuContext);
 DECLARE_bool(abandonGpuContext);
 DECLARE_string(skps);
 DECLARE_int32(threads);
 DECLARE_string(resourcePath);
 DECLARE_bool(verbose);
 DECLARE_bool(veryVerbose);
 DECLARE_string(writePath);
 
 DECLARE_string(key);
 DECLARE_string(properties);
 
+namespace SkCommonFlags {
+    /**
+     *  Helper to assist in collecting images from --images.
+     *
+     *  Populates an input array of strings with paths to images to test.
+     *
+     *  If a directory is passed to --images that does not have any supported images (based on file
+     *  type), this will exit the program, so the user or bot can rerun using the correct directory.
+     *
+     *  If a file is passed to --images, assume the user is deliberately testing this image, regardless
+     *  of file type.
+     */
+    void collectImages(SkTArray<SkString>*);
+}
+
 #endif