RTCVDAdapter: Always delegate feature value in Vp9HWSupportForSL()

Originally RTCVideoDecoderAdapter::Vp9HWSupportForSpatialLayers()
returns media::kVaapiVp9kSVCHWDecoding feature value on ChromeOS x86
devices and otherwise false.

Some ChromeOS ARM devices, but not all of them, are capable of
decoding SVC streams. So we need to handle this within a platform
VideoDecoders for ChromeOS ARM; if a VideoDecoder detects a VP9 SVC
stream is not supported, then it triggers a software decoder
fallback.

This CL changes the Vp9HWSupportForSpatialLayers() so that it returns
media::kVaapiVp9kSVCHWDecoding on all platforms. It enables keeping
the feature disabled on non ChromeOS platform and relies ChromeOS
video decoders for the judgement. This also drops "Vaapi" from the
feature name.

Bug: b:203747982
Test: Meet on volteer and trogdor

Change-Id: I422264229043e27b73022eb9bd65e6ad688cc046
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3230665
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#936257}

• media/base/media_switches.cc[diff]
• media/base/media_switches.h[diff]
• media/gpu/test/video_player/video_player_test_environment.cc[diff]
• media/gpu/v4l2/v4l2_stateful_workaround.cc[diff]
• media/gpu/vp9_decoder.cc[diff]
• third_party/blink/renderer/platform/peerconnection/rtc_video_decoder_adapter.cc[diff]