Wiring the RTCRtpEncodingParameters.priority down to the bitrate allocator.

I followed the wiring path for the max bitrate.
Doc:
https://docs.google.com/a/google.com/document/d/1sGT6y00prOIErFuGD44zWZacDpR6Rkjg_HXA_Z3Vw4Q/edit?usp=sharing

Bug: webrtc:8630
Change-Id: I6b861816670442656721c20f81d035ee5eb6218c
Reviewed-on: https://webrtc-review.googlesource.com/30380
Commit-Queue: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Peter Thatcher <pthatcher@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21397}

test/encoder_settings.cc

@@ -55,6 +55,7 @@ std::vector<VideoStream> CreateVideoStreams(
 
   stream_settings[encoder_config.number_of_streams - 1].max_bitrate_bps +=
       bitrate_left_bps;
+  stream_settings[0].bitrate_priority = encoder_config.bitrate_priority;
 
   return stream_settings;
 }