Remove RTPPayloadStrategy and simplify RTPPayloadRegistry

This CL removes RTPPayloadStrategy that is currently used to handle
audio/video specific aspects of payload handling. Instead, the audio and
video specific aspects will now have different functions, with linear
code flow.

This CL does not contain any functional changes, and is just a
preparation for future CL:s.

The main purpose with this CL is to add this function:
bool PayloadIsCompatible(const RtpUtility::Payload& payload,
                         const webrtc::VideoCodec& video_codec);
that can easily be extended in a future CL to look at video codec
specific information.

BUG=webrtc:6743

Review-Url: https://codereview.webrtc.org/2524923002
Cr-Commit-Position: refs/heads/master@{#15232}

webrtc/modules/video_coding/test/rtp_player.cc

@@ -271,8 +271,7 @@ class SsrcHandlers {
             const PayloadTypes& payload_types,
             LostPackets* lost_packets)
         : rtp_header_parser_(RtpHeaderParser::Create()),
-          rtp_payload_registry_(new RTPPayloadRegistry(
-              RTPPayloadStrategy::CreateStrategy(false))),
+          rtp_payload_registry_(new RTPPayloadRegistry()),
           rtp_module_(),
           payload_sink_(),
           ssrc_(ssrc),