Fixes issue triggered by WebRTC-VP9-PerformanceFlags trial.

Using WebRTC-VP9-PerformanceFlags and settings a multi-layer config,
and then configuring the codec in non-svc mode would cause us to not
set the cpu speed in libvpx. For some reason, that could trigger a
crash in the encoder.

This CL fixes that, and adds new test coverage for the code affected
byt the trial.

Bug: chromium:1167353, webrtc:11551
Change-Id: Iddb92fe03fc12bac37717908a8b5df4f3d411bf2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/202761
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33051}

modules/video_coding/BUILD.gn

@@ -866,6 +866,7 @@ if (rtc_include_tests) {
       ":video_coding_utility",
       ":videocodec_test_impl",
       ":webrtc_h264",
+      ":webrtc_libvpx_interface",
       ":webrtc_multiplex",
       ":webrtc_vp8",
       ":webrtc_vp9",
@@ -892,6 +893,7 @@ if (rtc_include_tests) {
       "../../media:rtc_simulcast_encoder_adapter",
       "../../media:rtc_vp9_profile",
       "../../rtc_base",
+      "../../test:explicit_key_value_config",
       "../../test:field_trial",
       "../../test:fileutils",
       "../../test:test_support",