Add support for a forced software encoder fallback.

Make it possible to switch from VP8 HW -> VP8 SW -> VP8 HW depending on bitrate and resolution.

BUG=webrtc:6634

Review-Url: https://codereview.webrtc.org/2988963002
Cr-Commit-Position: refs/heads/master@{#19362}

webrtc/modules/video_coding/codecs/vp8/default_temporal_layers.cc

@@ -19,6 +19,7 @@
 #include "webrtc/modules/video_coding/codecs/vp8/include/vp8_common_types.h"
 #include "webrtc/modules/video_coding/include/video_codec_interface.h"
 #include "webrtc/rtc_base/checks.h"
+#include "webrtc/system_wrappers/include/field_trial.h"
 
 #include "vpx/vpx_encoder.h"
 #include "vpx/vp8cx.h"
@@ -248,6 +249,16 @@ std::vector<TemporalLayers::FrameConfig> GetTemporalPattern(size_t num_layers) {
       TemporalLayers::kNone, TemporalLayers::kNone, TemporalLayers::kNone)};
 }
 
+// Temporary fix for forced SW fallback.
+// For VP8 SW codec, |TemporalLayers| is created and reported to
+// SimulcastRateAllocator::OnTemporalLayersCreated but not for VP8 HW.
+// Causes an issue when going from forced SW -> HW as |TemporalLayers| is not
+// deregistred when deleted by SW codec (tl factory might not exist, owned by
+// SimulcastRateAllocator).
+bool ExcludeOnTemporalLayersCreated(int num_temporal_layers) {
+  return webrtc::field_trial::IsEnabled("WebRTC-VP8-Forced-Fallback-Encoder") &&
+         num_temporal_layers == 1;
+}
 }  // namespace
 
 DefaultTemporalLayers::DefaultTemporalLayers(int number_of_temporal_layers,
@@ -372,7 +383,7 @@ TemporalLayers* TemporalLayersFactory::Create(
     uint8_t initial_tl0_pic_idx) const {
   TemporalLayers* tl =
       new DefaultTemporalLayers(temporal_layers, initial_tl0_pic_idx);
-  if (listener_)
+  if (listener_ && !ExcludeOnTemporalLayersCreated(temporal_layers))
     listener_->OnTemporalLayersCreated(simulcast_id, tl);
   return tl;
 }