[Media] Remove shipped features.

This CL removes the following base::Features and associated UI flags:

- kVp9kSVCHWDecoding which shipped on ChromeOS x86 in M97
- kVaapiVp9kSVCHWEncoding which shipped on ChromeOS x86 in 11/2021,
  and is possibly shipping on Windows D3D11 in the future

Because these features are only shipped on certain platforms,
to remove the features, I added platform_features.h to allow code
to query for platform-specific feature support.

Cq-Include-Trybots: luci.chromium.try:chromeos-arm-generic-rel,chromeos-arm-generic-dbg,chromeos-arm64-generic-rel,chromeos-jacuzzi-rel;luci.chrome.try:chromeos-jacuzzi-chrome
Bug: 1325698,1186051
Fixed: 1419256,1419258
Change-Id: Ic7b8403c18146830dfef619a88e01b39fb2806b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4953598
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: Mark Foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1214275}

• chrome/browser/about_flags.cc[diff]
• chrome/browser/flag-metadata.json[diff]
• chrome/browser/flag_descriptions.cc[diff]
• chrome/browser/flag_descriptions.h[diff]
• media/base/BUILD.gn[diff]
• media/base/media_switches.cc[diff]
• media/base/media_switches.h[diff]
• media/base/platform_features.cc[Added - diff]
• media/base/platform_features.h[Added - diff]
• media/gpu/test/video_encoder/video_encoder_test_environment.cc[diff]
• media/gpu/v4l2/v4l2_video_decoder_backend_stateful.cc[diff]
• media/gpu/vaapi/vaapi_unittest.cc[diff]
• media/gpu/vaapi/vaapi_video_encode_accelerator.cc[diff]
• media/gpu/vaapi/vaapi_wrapper.cc[diff]
• media/gpu/vp9_decoder.cc[diff]
• third_party/blink/renderer/platform/peerconnection/rtc_video_decoder_adapter.cc[diff]
• third_party/blink/renderer/platform/peerconnection/rtc_video_decoder_adapter.h[diff]
• third_party/blink/renderer/platform/peerconnection/rtc_video_decoder_adapter_test.cc[diff]
• third_party/blink/renderer/platform/peerconnection/rtc_video_decoder_factory.cc[diff]
• third_party/blink/renderer/platform/peerconnection/rtc_video_decoder_factory_test.cc[diff]
• third_party/blink/renderer/platform/peerconnection/rtc_video_decoder_stream_adapter.cc[diff]
• third_party/blink/renderer/platform/peerconnection/rtc_video_decoder_stream_adapter_test.cc[diff]
• third_party/blink/renderer/platform/peerconnection/rtc_video_encoder.cc[diff]