Fix DecoderStream::ReinitializeDecoder to always pass in the CdmContext.

Also added unittests to cover the case where neither DecryptionVideoDecoder nor
DecryptingDemuxerStream is selected for encrypted config, but another
VideoDecoder that supports encrypted config is selected, initialized, and
reinitialized. In practice, this could be the case when the CDM doesn't support
the Decryptor interface, e.g. MediaDrmBridge on Android.

BUG=679095

Review-Url: https://codereview.chromium.org/2650823006
Cr-Commit-Position: refs/heads/master@{#446190}