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/modules/video_coding/codecs/vp8/default_temporal_layers_unittest.cc b/modules/video_coding/codecs/vp8/default_temporal_layers_unittest.cc
index f61c302..2b69745 100644
--- a/modules/video_coding/codecs/vp8/default_temporal_layers_unittest.cc
+++ b/modules/video_coding/codecs/vp8/default_temporal_layers_unittest.cc
@@ -10,8 +10,8 @@
 
 #include "modules/video_coding/codecs/vp8/default_temporal_layers.h"
 #include "modules/video_coding/codecs/vp8/libvpx_vp8_encoder.h"
-#include "modules/video_coding/codecs/vp8/simulcast_rate_allocator.h"
 #include "modules/video_coding/include/video_codec_interface.h"
+#include "modules/video_coding/utility/simulcast_rate_allocator.h"
 #include "test/field_trial.h"
 #include "test/gtest.h"