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

webrtc/modules/video_coding/BUILD.gn

Index: webrtc/modules/video_coding/BUILD.gn
diff --git a/webrtc/modules/video_coding/BUILD.gn b/webrtc/modules/video_coding/BUILD.gn
index 4836207a9f202361f5dd12a3a83a634383fa7288..45e2873a5e8e9c6ddde21f746a97f4f2e02dbce7 100644
--- a/webrtc/modules/video_coding/BUILD.gn
+++ b/webrtc/modules/video_coding/BUILD.gn
@@ -31,6 +31,7 @@ rtc_static_library("video_coding") {
     "generic_encoder.h",
     "histogram.cc",
     "histogram.h",
+    "include/video_bitrate_allocator_factory.h",
     "include/video_coding.h",
     "include/video_coding_defines.h",
     "inter_frame_delay.cc",
@@ -68,6 +69,7 @@ rtc_static_library("video_coding") {
     "timestamp_map.h",
     "timing.cc",
     "timing.h",
+    "video_bitrate_allocator_factory.cc",
     "video_coding_impl.cc",
     "video_coding_impl.h",
     "video_receiver.cc",
@@ -96,6 +98,8 @@ rtc_static_library("video_coding") {
 
 rtc_static_library("video_coding_utility") {
   sources = [
+    "utility/default_video_bitrate_allocator.cc",
+    "utility/default_video_bitrate_allocator.h",
     "utility/frame_dropper.cc",
     "utility/frame_dropper.h",
     "utility/h264_bitstream_parser.cc",