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

tools/flags/SkCommonFlags.h

Index: tools/flags/SkCommonFlags.h
diff --git a/tools/flags/SkCommonFlags.h b/tools/flags/SkCommonFlags.h
index 127d37328eb1b05981e72414ce94a33e837a6b27..8e00e7516c14419d8fee650e540fc38d96fe6cc9 100644
--- a/tools/flags/SkCommonFlags.h
+++ b/tools/flags/SkCommonFlags.h
@@ -9,6 +9,8 @@
 #define SK_COMMON_FLAGS_H
 
 #include "SkCommandLineFlags.h"
+#include "SkTArray.h"
+#include "SkString.h"
 
 DECLARE_bool(cpu);
 DECLARE_bool(dryRun);
@@ -30,4 +32,18 @@ DECLARE_bool(pre_log);
 DECLARE_string(key);
 DECLARE_string(properties);
 
+/**
+ *  Helper to assist in collecting image paths from --images.
+ *
+ *  Populates an array of strings with paths to images to test.
+ *
+ *  Returns true if each argument to --images is meaningful:
+ *  - If the file/directory does not exist, return false.
+ *  - If a directory passed to --images does not have any supported images (based on file
+ *  type), return false.
+ *  - If a file is passed to --images, assume the user is deliberately testing this image,
+ *  regardless of file type.
+ */
+bool CollectImages(SkTArray<SkString>*);
+
 #endif