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 ddd8d3a4a2f86022becfefcbdbad9f3fae604048..f1aa1f398751453d89a8e8fb5fbd556ba3fe6c60 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);
@@ -29,4 +31,19 @@ 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