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

gyp/flags.gyp

Index: gyp/flags.gyp
diff --git a/gyp/flags.gyp b/gyp/flags.gyp
index 769a7bdb57721d1b35e212b7b41335c52006ae38..e3b850cf65d4e227d37f655f8a6c9d50628bbd93 100644
--- a/gyp/flags.gyp
+++ b/gyp/flags.gyp
@@ -39,6 +39,13 @@
         '../tools/flags/SkCommonFlagsConfig.cpp',
         '../tools/flags/SkCommonFlagsConfig.h',
       ],
+      'conditions':[
+        # FIXME: We redefine this here because this target does not depend on raw_codec, which
+        # defines it.
+        ['skia_codec_decodes_raw', {
+          'defines': [ 'SK_CODEC_DECODES_RAW' ],
+        }],
+      ],
       'dependencies': [
         'skia_lib.gyp:skia_lib',
         'flags.gyp:flags',