EME: Address TODOs related to the removal of prefixed EME

media/cdm/ppapi/external_clear_key/clear_key_cdm.cc

Index: media/cdm/ppapi/external_clear_key/clear_key_cdm.cc
diff --git a/media/cdm/ppapi/external_clear_key/clear_key_cdm.cc b/media/cdm/ppapi/external_clear_key/clear_key_cdm.cc
index 7939dc964d06d3e0ccb9d2ac6061568488d884d5..5367f5613a6953c8f830de0118c20e7a5b51659e 100644
--- a/media/cdm/ppapi/external_clear_key/clear_key_cdm.cc
+++ b/media/cdm/ppapi/external_clear_key/clear_key_cdm.cc
@@ -335,9 +335,11 @@ void ClearKeyCdm::LoadSession(uint32_t promise_id,
                      promise_id),
           base::Bind(&ClearKeyCdm::OnPromiseFailed, base::Unretained(this),
                      promise_id)));
+  // AesDecryptor does not support loading, but we need a real session so create

[ddorwin, 2016/08/11 20:28:32]

Is this comment accurate?

[jrummell, 2016/08/12 19:24:25]

How about "AesDecryptor does not support loading, so create a temporary session
with fake initData to represent it."?

[ddorwin, 2016/08/12 21:40:38]

Done.

+  // one with fake initData.
   decryptor_->CreateSessionAndGenerateRequest(
       MediaKeys::TEMPORARY_SESSION, EmeInitDataType::WEBM,
-      std::vector<uint8_t>(), std::move(promise));
+      std::vector<uint8_t>(16), std::move(promise));

[ddorwin, 2016/08/11 20:28:32]

Made this 128 (random) bits. 0 bytes failed the check in aes_decryptor.cc.

[jrummell, 2016/08/12 19:24:25]

The keyId used is kLoadableSessionKeyId. So it could be used here, but it is
ignored in OnSessionMessage. However, it might be more explicit if that key was
used here (since that is the intent).

[ddorwin, 2016/08/12 21:40:38]

Done.

 }
 
 void ClearKeyCdm::UpdateSession(uint32_t promise_id,