Remove webrtc::VideoEncoderFactory

Replace the use of webrtc::VideoEncoderFactory with
cricket::WebRtcVideoEncoderFactory and remove the adapter classes
between these two factory types.

Some code changes were necessary in order to accomplish this:
 * Move SimulcastEncoderAdapter from
   webrtc/modules/video_coding/codecs/vp8 to webrtc/media/engine (that's
   where it's used).
 * Rename simulcast_unittest.h to simulcast_test_utility.h and make it
   into it's own target, because it's used from both
   simulcast_unittest.cc and simulcast_encoder_adapter_unittest.cc.
 * Remove ownership of the encoder factory from SimulcastEncoderAdapter,
   and make the necessary changes in surrounding code.

The goal with this CL is to clean up the code, and also to free up
the name webrtc::VideoEncoderFactory for future use.

BUG=webrtc:7925

Review-Url: https://codereview.webrtc.org/2964953002
Cr-Commit-Position: refs/heads/master@{#18945}
diff --git a/webrtc/media/BUILD.gn b/webrtc/media/BUILD.gn
index c9055c8..fad410b 100644
--- a/webrtc/media/BUILD.gn
+++ b/webrtc/media/BUILD.gn
@@ -143,6 +143,8 @@
     "engine/payload_type_mapper.h",
     "engine/simulcast.cc",
     "engine/simulcast.h",
+    "engine/simulcast_encoder_adapter.cc",
+    "engine/simulcast_encoder_adapter.h",
     "engine/videodecodersoftwarefallbackwrapper.cc",
     "engine/videodecodersoftwarefallbackwrapper.h",
     "engine/videoencodersoftwarefallbackwrapper.cc",
@@ -228,6 +230,7 @@
     "../base:rtc_base",
     "../base:rtc_base_approved",
     "../base:rtc_task_queue",
+    "../base:sequenced_task_checker",
     "../call",
     "../common_video:common_video",
     "../modules/audio_coding:rent_a_codec",
@@ -436,6 +439,7 @@
       "engine/internaldecoderfactory_unittest.cc",
       "engine/nullwebrtcvideoengine_unittest.cc",
       "engine/payload_type_mapper_unittest.cc",
+      "engine/simulcast_encoder_adapter_unittest.cc",
       "engine/simulcast_unittest.cc",
       "engine/videodecodersoftwarefallbackwrapper_unittest.cc",
       "engine/videoencodersoftwarefallbackwrapper_unittest.cc",
@@ -513,6 +517,7 @@
       "../logging:rtc_event_log_api",
       "../modules/audio_device:mock_audio_device",
       "../modules/audio_processing:audio_processing",
+      "../modules/video_coding:simulcast_test_utility",
       "../modules/video_coding:video_coding_utility",
       "../modules/video_coding:webrtc_vp8",
       "../p2p:p2p_test_utils",