Revert CPUUSED back to '6' for VP9 in CRD

Details are in the bug but basically I see a decent increase in network
traffic for certain encoding workloads. There is also some variability
in the performance gains which means it's safer to revert back to the
original value used for CPUUSED in VP9 and then add a client-side toggle
to allow users to adjust this value for their network and workload.

(cherry picked from commit a28f874)

Bug: 1328377
Change-Id: I5bbde5c20d6c72b5c138c71d75f59d89c85d6146
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3661377
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1006591}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3665421
Cr-Commit-Position: refs/branch-heads/5060@{#227}
Cr-Branched-From: b83393d-refs/heads/main@{#1002911}

remoting/codec/webrtc_video_encoder_vpx.cc

@@ -143,7 +143,7 @@ void SetVp9CodecOptions(vpx_codec_ctx_t* codec, bool lossless_encode) {
   // Request the lowest-CPU usage that VP9 supports, which depends on whether
   // we are encoding lossy or lossless.
   // Note that this knob uses the same parameter name as VP8.
-  int cpu_used = lossless_encode ? 5 : 8;
+  int cpu_used = lossless_encode ? 5 : 6;
   vpx_codec_err_t ret = vpx_codec_control(codec, VP8E_SET_CPUUSED, cpu_used);
   DCHECK_EQ(VPX_CODEC_OK, ret) << "Failed to set CPUUSED";