Chromecast PSSH boxes must use data_size parameter.

chromecast/media/cdm/chromecast_init_data.cc

Index: chromecast/media/cdm/chromecast_init_data.cc
diff --git a/chromecast/media/cdm/chromecast_init_data.cc b/chromecast/media/cdm/chromecast_init_data.cc
index ba27d2efd95acc14069b03ca950094e0c87af6cd..d9e76a51b11dd758cc9c287149c6ee8643a342de 100644
--- a/chromecast/media/cdm/chromecast_init_data.cc
+++ b/chromecast/media/cdm/chromecast_init_data.cc
@@ -107,6 +107,11 @@ bool FindChromecastInitData(const std::vector<uint8_t>& init_data,
     RCHECK(reader.SkipBits(16 * 8));
     box_bytes_read += 16;
 
+    uint32_t data_size;
+    RCHECK(reader.ReadBits(4 * 8, &data_size));
+    box_bytes_read += 4;
+    RCHECK(data_size == box_size - box_bytes_read);

[lcwu1, 2015/06/09 18:22:22]

Are we adding a new data_size field in the custom data? Why do we need this?

I am fine with that, but if so, we will need to communicate that with devrel and
tam teams. 

[gunsch, 2015/06/09 18:54:13]

We are not, and even if we were this does not affect anything for content
partners.

1) The way we communicate CustomData to our plugin is an internal implementation
detail negotiated between MPL and cast_shell.

2) PSSH boxes *must* have a four-byte data_size field (per the CENC spec) at
this location, even though that duplicates information about the rest of the
box. So this was actually a bug in my first implementation---not realizing this
difference about PSSH boxes in particular as opposed to the UUID box we were
using previously.

+
     uint16_t msg_type;
     RCHECK(reader.ReadBits(2 * 8, &msg_type));
     box_bytes_read += 2;