Refactor RTCP sender

The main purpose of this CL is to clean up RTCPSender::PrepareRTCP, but
it has quite a few ramifications. Notable changes:

* Removed the rtcpPacketTypeFlags bit vector and don't assume
  RTCPPacketType values have a single unique bit set. This will allow
  making this an enum class once rtcp_receiver has been overhauled.

* Flags are now stored in a map that is a member of the class. This
  meant we could remove some bool flags (eg send_remb_) which was
  previously masked into rtcpPacketTypeFlags and then masked out again
  when testing if a remb packet should be sent.

* Make all build methods, eg. BuildREMB(), have the same signature.
  An RtcpContext struct was introduced for this purpose. This allowed
  the use of a map from RTCPPacketType to method pointer. Instead of
  18 consecutive if-statements, there is now a single loop.
  The context class also allowed some simplifications in the build
  methods themselves.

* A few minor simplifications and cleanups.

The next step is to gradually replace the builder methods with the
builders from the new RtcpPacket classes.

BUG=2450
R=asapersson@webrtc.org, pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/48329004

Cr-Commit-Position: refs/heads/master@{#9166}

webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h

@@ -161,7 +161,10 @@ class ModuleRtpRtcpImpl : public RtpRtcp {
 
   // Force a send of an RTCP packet.
   // Normal SR and RR are triggered via the process function.
-  int32_t SendRTCP(uint32_t rtcp_packet_type = kRtcpReport) override;
+  int32_t SendRTCP(RTCPPacketType rtcpPacketType) override;
+
+  int32_t SendCompoundRTCP(
+      const std::set<RTCPPacketType>& rtcpPacketTypes) override;
 
   int32_t ResetSendDataCountersRTP() override;