media/gpu/vaapi: Add VP9 Encoder

Add the Hardware Accelerated VA-API based VP9 Encoder support
in chromium.

The vp9 specific webrtc metadata to make the encoding work with
appr.tc is added by  Erik SprÃ¥ng<[email protected]>.

BUG=811912
TEST=run the video_encode_accelerator_unittest on supported devices(eg: soraka)
VEA:
./video_encode_accelerator_unittest --gtest_filter=SimpleEncode/VideoEncodeAcceleratorTest.TestSimpleEncode/0 --test_stream_data=bear_320x192_40frames.yuv:320:192:12:bear.ivf:100000:30  -v=0 --disable_flush --single-process-tests --ozone-platform=gbm

TEST=appr.tc/?debug=loopback&vsc=VP9&vrc=VP9

Change-Id: I50b77d642684a03f7ecc35befd5442d35b41e3ae
Reviewed-on: https://chromium-review.googlesource.com/c/1383540
Reviewed-by: Avi Drissman <[email protected]>
Reviewed-by: Hirokazu Honda <[email protected]>
Reviewed-by: Becca Hughes <[email protected]>
Commit-Queue: Sreerenj Balachandran <[email protected]>
Cr-Commit-Position: refs/heads/master@{#630487}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index fe02ca6..209cb7c 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1258,6 +1258,9 @@
 extern const char kWebrtcHwVP8EncodingName[];
 extern const char kWebrtcHwVP8EncodingDescription[];
 
+extern const char kWebrtcHwVP9EncodingName[];
+extern const char kWebrtcHwVP9EncodingDescription[];
+
 extern const char kWebrtcNewEncodeCpuLoadEstimatorName[];
 extern const char kWebrtcNewEncodeCpuLoadEstimatorDescription[];