Avoid race in MSE_Widevine tests

When run with the stub Widevine CDM, these tests suffer from a race
between the result of a failed StubCdm::Decrypt() and a failed
StubCdm::UpdateSession().  The tests expect the latter, but the former
often arrives first.

By allowing the stub to InitializeVideoDecoder() successfully and
returning cdm::kNoKey upon DecryptAndDecodeFrame() attempt we avoid the
DecryptingDemuxerStream errors altogether, so there is no race anymore.

Bug: 768745
Test: 'browser_tests --gtest_filter=*MSE_Widevine*' pass when run with the stub Widevine CDM

Change-Id: I35a35d006d118f8f7f9a8a64cf9334017644f4ea
Reviewed-on: https://chromium-review.googlesource.com/831026Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Wojciech Dzierżanowski <wdzierzanowski@opera.com>
Cr-Commit-Position: refs/heads/master@{#524663}

media/cdm/stub/stub_cdm.cc

@@ -114,12 +114,12 @@ cdm::Status StubCdm::Decrypt(const cdm::InputBuffer& /* encrypted_buffer */,
 
 cdm::Status StubCdm::InitializeAudioDecoder(
     const cdm::AudioDecoderConfig& /* audio_decoder_config */) {
-  return cdm::kDecryptError;
+  return cdm::kSuccess;
 }
 
 cdm::Status StubCdm::InitializeVideoDecoder(
     const cdm::VideoDecoderConfig& /* video_decoder_config */) {
-  return cdm::kDecryptError;
+  return cdm::kSuccess;
 }
 
 void StubCdm::ResetDecoder(cdm::StreamType /* decoder_type */) {
@@ -131,13 +131,13 @@ void StubCdm::DeinitializeDecoder(cdm::StreamType /* decoder_type */) {
 cdm::Status StubCdm::DecryptAndDecodeFrame(
     const cdm::InputBuffer& /* encrypted_buffer */,
     cdm::VideoFrame* /* decoded_frame */) {
-  return cdm::kDecryptError;
+  return cdm::kNoKey;
 }
 
 cdm::Status StubCdm::DecryptAndDecodeSamples(
     const cdm::InputBuffer& /* encrypted_buffer */,
     cdm::AudioFrames* /* audio_frames */) {
-  return cdm::kDecryptError;
+  return cdm::kNoKey;
 }
 
 void StubCdm::Destroy() {