Encrypted Media: DecryptingXxx::SetDecryptor() accepts NULL decryptor.

Currently ProxyDecryptor never sets NULL decryptor in DecryptorReadyCB. However on Chrome on Android, we have a legit use case of this. When ProxyDecryptor cannot create a decryptor for the specified key system, it'll choose to go through the IPC route to defer decryption to the browser. In this case, it could set NULL decryptor in DecrypingDemuxerStream's SetDecryptor() callback to trigger DDS init failure, so that MediaSourceDelegate will not try to decrypt the buffers at the renderer side. Instead it'll send all encrypted buffers over the IPC to the browser.

BUG=163552

Review URL: https://chromiumcodereview.appspot.com/17422002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@207202 0039d316-1c4b-4281-b951-d872f2087c98

• media/filters/decrypting_audio_decoder.cc[diff]
• media/filters/decrypting_audio_decoder_unittest.cc[diff]
• media/filters/decrypting_demuxer_stream.cc[diff]
• media/filters/decrypting_demuxer_stream_unittest.cc[diff]
• media/filters/decrypting_video_decoder.cc[diff]
• media/filters/decrypting_video_decoder_unittest.cc[diff]