Reland of Issue 2434073003: Extract bitrate allocation ...

This is a reland of https://codereview.webrtc.org/2434073003/ including
some fixes for failing test cases.

Original description:

Extract bitrate allocation of spatial/temporal layers out of codec impl.

This CL makes a number of intervowen changes:

* Add BitrateAllocation struct, that contains a codec independent view
  of how the target bitrate is distributed over spatial and temporal
  layers.

* Adds the BitrateAllocator interface, which takes a bitrate and frame
  rate and produces a BitrateAllocation.

* A default (non layered) implementation is added, and
  SimulcastRateAllocator is extended to fully handle VP8 allocation.
  This includes capturing TemporalLayer instances created by the
  encoder.

* ViEEncoder now owns both the bitrate allocator and the temporal layer
  factories for VP8. This allows allocation to happen fully outside of
  the encoder implementation.

This refactoring will make it possible for ViEEncoder to signal the
full picture of target bitrates to the RTCP module.

BUG=webrtc:6301

Review-Url: https://codereview.webrtc.org/2488833004
Cr-Commit-Position: refs/heads/master@{#15023}
diff --git a/webrtc/modules/video_coding/video_coding_impl.h b/webrtc/modules/video_coding/video_coding_impl.h
index cb24654..0533634 100644
--- a/webrtc/modules/video_coding/video_coding_impl.h
+++ b/webrtc/modules/video_coding/video_coding_impl.h
@@ -34,6 +34,8 @@
 
 namespace webrtc {
 
+class VideoBitrateAllocator;
+
 namespace vcm {
 
 class VCMProcessTimer {
@@ -75,9 +77,15 @@
   int Bitrate(unsigned int* bitrate) const;
   int FrameRate(unsigned int* framerate) const;
 
-  int32_t SetChannelParameters(uint32_t target_bitrate,  // bits/s.
+  int32_t SetChannelParameters(uint32_t target_bitrate_bps,
                                uint8_t lossRate,
-                               int64_t rtt);
+                               int64_t rtt,
+                               VideoBitrateAllocator* bitrate_allocator);
+  // Calls SetChannelParameters(), with the previous target bitrate, loss rate
+  // and RTT, but reallocates the bitrate allocation based on a presumably
+  // updated codec configuration.
+  int32_t UpdateChannelParemeters(VideoBitrateAllocator* bitrate_allocator);
+
   // Deprecated:
   // TODO(perkj): Remove once no projects use it.
   int32_t RegisterProtectionCallback(VCMProtectionCallback* protection);