Consolidate mock video encoders

Use the MockVideoEncoder from the api target in
encoder_simulcast_proxy_unittest, rather than a custom MockEncoder.
This also prevents issue when new SetRates() is made pure virtual.

Bug: webrtc:10481
Change-Id: I72469803c00f7014eeac5b9321d1e0d716fa245d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/134211
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27774}
diff --git a/media/BUILD.gn b/media/BUILD.gn
index fbed41d..f28461e 100644
--- a/media/BUILD.gn
+++ b/media/BUILD.gn
@@ -509,6 +509,7 @@
       "../api:mock_video_bitrate_allocator",
       "../api:mock_video_bitrate_allocator_factory",
       "../api:mock_video_codec_factory",
+      "../api:mock_video_encoder",
       "../api:scoped_refptr",
       "../api:simulcast_test_fixture_api",
       "../api/audio_codecs:builtin_audio_decoder_factory",