media: Enable VP9 encoder on ChromeOS x86

Bug: 811912
Test: HW encoder is used on https://appr.tc/?debug=loopback&vsc=vp9&vrc=vp9
Test: video.EncodeAccel.vp9*
Test: webrtc.*.vp9*
Change-Id: If0457bac7e06488b54b4fd77e7b7e8e766da3352
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2298883Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797124}

media/base/media_switches.cc

@@ -431,7 +431,7 @@ const base::Feature kVaapiVP8Encoder{"VaapiVP8Encoder",
 
 // Enable VA-API hardware encode acceleration for VP9.
 const base::Feature kVaapiVP9Encoder{"VaapiVP9Encoder",
-                                     base::FEATURE_DISABLED_BY_DEFAULT};
+                                     base::FEATURE_ENABLED_BY_DEFAULT};
 
 #if defined(ARCH_CPU_X86_FAMILY) && defined(OS_CHROMEOS)
 // Enable VP9 k-SVC decoding with HW decoder for webrtc use case on ChromeOS.