Revert "Enable VA-API-based HW VP8 encoding by default."

This reverts commit 7528f618.

Reason for revert: disable the feature by default while related issues are being investigated (b/111781384).

Original change's description:
> Enable VA-API-based HW VP8 encoding by default.
>
> Flip the VaapiVP8Encoder flag to enable VA-API based HW VP8 encoder by
> default.
>
> BUG=794608
> TEST=veatest, apprtc
>
> Change-Id: If99f825788786dc4cc2be8c4f7a3908d04023368
> Reviewed-on: https://chromium-review.googlesource.com/1080436
> Reviewed-by: Dan Sanders <sandersd@chromium.org>
> Commit-Queue: Pawel Osciak <posciak@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#564004}

TBR=posciak@chromium.org,xhwang@chromium.org,sandersd@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 794608,b:111781384
Change-Id: I35232863ca5b96285994cb03553548b0b72ff169
Reviewed-on: https://chromium-review.googlesource.com/1160181Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Pawel Osciak <posciak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580402}

media/base/media_switches.cc

@@ -327,7 +327,7 @@ const base::Feature kUseSurfaceLayerForVideo{"UseSurfaceLayerForVideo",
 
 // Enable VA-API hardware encode acceleration for VP8.
 const base::Feature kVaapiVP8Encoder{"VaapiVP8Encoder",
-                                     base::FEATURE_ENABLED_BY_DEFAULT};
+                                     base::FEATURE_DISABLED_BY_DEFAULT};
 
 // Inform video blitter of video color space.
 const base::Feature kVideoBlitColorAccuracy{"video-blit-color-accuracy",