fix(MSS): Fix playback of some MSS streams (#7517)

With this change, no default flags are used. It seems that when a sample
has to use the default flags, in Safari, it can cause problems with MSS.
With HLS this does not happen.

Fixes #7509

lib/util/mp4_generator.js

@@ -765,7 +765,7 @@ shaka.util.Mp4Generator = class {
       0x00, 0x00, 0x00, 0x01, // default_sample_description_index
       0x00, 0x00, 0x00, 0x00, // default_sample_duration
       0x00, 0x00, 0x00, 0x00, // default_sample_size
-      0x00, 0x01, 0x00, 0x01, // default_sample_flags
+      0x00, 0x00, 0x00, 0x00, // default_sample_flags
     ]);
     return Mp4Generator.box('trex', bytes);
   }