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 9bd746be75846cb67a3a5ae274867c1a0472b8a9..8f413d9820c97515e0dd8ddb8cce915b75294964 100644
--- a/webrtc/modules/video_coding/BUILD.gn
+++ b/webrtc/modules/video_coding/BUILD.gn
@@ -33,6 +33,7 @@ rtc_static_library("video_coding") {
     "h264_sps_pps_tracker.h",
     "histogram.cc",
     "histogram.h",
+    "include/video_codec_initializer.h",
     "include/video_coding.h",
     "include/video_coding_defines.h",
     "inter_frame_delay.cc",
@@ -70,6 +71,7 @@ rtc_static_library("video_coding") {
     "timestamp_map.h",
     "timing.cc",
     "timing.h",
+    "video_codec_initializer.cc",
     "video_coding_impl.cc",
     "video_coding_impl.h",
     "video_receiver.cc",
@@ -98,6 +100,8 @@ 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/ivf_file_writer.cc",