Break out allocation from BitrateController into a BitrateAllocator.

This also refactors some of the padding and allocation code in ViEEncoder, and
makes ChannelGroup a simple forwarder from BitrateController to
BitrateAllocator.

This CL is part of a bigger picture, see https://review.webrtc.org/35319004/ for
details.

BUG=4323
R=mflodman@webrtc.org, pbos@webrtc.org, sprang@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44399004

Cr-Commit-Position: refs/heads/master@{#8595}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8595 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/webrtc/modules/bitrate_controller/BUILD.gn b/webrtc/modules/bitrate_controller/BUILD.gn
index 59f032b..af3b8dc 100644
--- a/webrtc/modules/bitrate_controller/BUILD.gn
+++ b/webrtc/modules/bitrate_controller/BUILD.gn
@@ -10,8 +10,10 @@
 
 source_set("bitrate_controller") {
   sources = [
+    "bitrate_allocator.cc",
     "bitrate_controller_impl.cc",
     "bitrate_controller_impl.h",
+    "include/bitrate_allocator.h",
     "include/bitrate_controller.h",
     "remb_suppressor.cc",
     "remb_suppressor.h",