Defines an abstract request signer class and two implementations of the
signer class, AesRequestSigner for AES-CBC signing and RsaRequestSigner
for RSA-PSS signing.
WidevineEncryptorSource now uses Signer for signature generation.
Change-Id: I4528409a9be998535bccde40fdadb412e4bbdaf3
Use EncryptorSource::CreateEncryptor for encryptor creation.
EncryptorSource no longer owns the created encryptor.
Change-Id: I34d1f32262b7692bcb347d3b084fd06cbae7850f
Kongqun Yang