Generalize SimulcastEncoderAdapter, use for H264 & VP8.

* Move SimulcastEncoderAdapter out under modules/video_coding
* Move SimulcastRateAllocator back out to modules/video_coding/utility
* Move TemporalLayers and ScreenshareLayers to modules/video_coding/utility
* Move any VP8 specific code - such as temporal layer bitrate budgeting -
  under codec type dependent conditionals.
* Plumb the simulcast index for H264 in the codec specific and RTP format data structures.

TBR=sprang@webrtc.org,stefan@webrtc.org,titovartem@webrtc.org

Bug: webrtc:5840
Change-Id: I2d3b130622dd7ceec5528f3ab6c46f109e6bafb8
Reviewed-on: https://webrtc-review.googlesource.com/84743
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23715}
diff --git a/media/BUILD.gn b/media/BUILD.gn
index 4b92d14..f932a89 100644
--- a/media/BUILD.gn
+++ b/media/BUILD.gn
@@ -191,10 +191,10 @@
     "../api/video_codecs:video_codecs_api",
     "../call:call_interfaces",
     "../call:video_stream_api",
+    "../modules/video_coding:video_coding_utility",
     "../modules/video_coding:webrtc_h264",
     "../modules/video_coding:webrtc_multiplex",
     "../modules/video_coding:webrtc_vp8",
-    "../modules/video_coding:webrtc_vp8_helpers",
     "../modules/video_coding:webrtc_vp9",
     "../rtc_base:checks",
     "../rtc_base:rtc_base_approved",
@@ -218,6 +218,7 @@
     "../modules/audio_processing/aec_dump:aec_dump",
     "../modules/video_coding:video_codec_interface",
     "../modules/video_coding:video_coding",
+    "../modules/video_coding:video_coding_utility",
     "../rtc_base:audio_format_to_string",
     "../rtc_base:base64",
     "../rtc_base:checks",
@@ -304,7 +305,6 @@
     "../modules/audio_mixer:audio_mixer_impl",
     "../modules/audio_processing:audio_processing",
     "../modules/video_capture:video_capture_module",
-    "../modules/video_coding:webrtc_vp8_helpers",
     "../pc:rtc_pc_base",
     "../rtc_base:rtc_base",
     "../rtc_base:rtc_task_queue",
@@ -474,6 +474,7 @@
       "../api/video:video_frame_i420",
       "../modules/audio_processing:mocks",
       "../modules/video_coding:video_codec_interface",
+      "../modules/video_coding:webrtc_vp8",
       "../pc:rtc_pc",
       "../pc:rtc_pc_base",
       "../rtc_base:checks",
@@ -569,7 +570,6 @@
       "../modules/audio_device:mock_audio_device",
       "../modules/audio_processing:audio_processing",
       "../modules/video_coding:simulcast_test_fixture_impl",
-      "../modules/video_coding:webrtc_vp8_helpers",
       "../p2p:p2p_test_utils",
       "../rtc_base:rtc_base",
       "../rtc_base:rtc_base_approved",