Remove the redundant method GetPayloadSpecifics

It's in the way of a refactoring.

Also change PayloadTypeToPayload---the method all callers can use instead---to return Optional<Payload> instead of const Payload* (for thread safety reasons: an object that protects itself with a mutex shouldn't be handing out pointers to parts of itself). 

BUG=webrtc:8159

Change-Id: I7ef0d545077ffdea016b309f2165e3c4955a2928
Reviewed-on: https://webrtc-review.googlesource.com/2360
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19917}
diff --git a/modules/rtp_rtcp/source/rtp_receiver_impl.cc b/modules/rtp_rtcp/source/rtp_receiver_impl.cc
index dd04614..040ece3 100644
--- a/modules/rtp_rtcp/source/rtp_receiver_impl.cc
+++ b/modules/rtp_rtcp/source/rtp_receiver_impl.cc
@@ -294,7 +294,7 @@
         if (rtp_header.payloadType == last_received_payload_type) {
           re_initialize_decoder = true;
 
-          const Payload* payload = rtp_payload_registry_->PayloadTypeToPayload(
+          const auto payload = rtp_payload_registry_->PayloadTypeToPayload(
               rtp_header.payloadType);
           if (!payload) {
             return;
@@ -382,7 +382,7 @@
         return 0;
       }
 
-      const Payload* payload =
+      const auto payload =
           rtp_payload_registry_->PayloadTypeToPayload(payload_type);
       if (!payload) {
         // Not a registered payload type.