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/1080436Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Pawel Osciak <posciak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564004}

media/base/media_switches.cc

@@ -304,7 +304,7 @@ const base::Feature kUseSurfaceLayerForVideo{"UseSurfaceLayerForVideo",
 
 // Enable VA-API hardware encode acceleration for VP8.
 const base::Feature kVaapiVP8Encoder{"VaapiVP8Encoder",
-                                     base::FEATURE_DISABLED_BY_DEFAULT};
+                                     base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Inform video blitter of video color space.
 const base::Feature kVideoBlitColorAccuracy{"video-blit-color-accuracy",