cros/video: Add constants for AGVDA histogram.

This entry is used to detect the HW video decoder is requested from
ARC containers and is successfully initialized.

Test=Run the cheets_mediaPlayerVideoHWDecodeUsed tests.
Bug=chromium:661864

Change-Id: Ibe45525d0edc2232c017a75dbe9e5c59f36ccc49
Reviewed-on: https://chromium-review.googlesource.com/424514
Commit-Ready: Owen Lin <owenlin@chromium.org>
Tested-by: Owen Lin <owenlin@chromium.org>
Reviewed-by: Owen Lin <owenlin@chromium.org>
diff --git a/client/cros/video/constants.py b/client/cros/video/constants.py
index c325e80..c521c9e 100644
--- a/client/cros/video/constants.py
+++ b/client/cros/video/constants.py
@@ -27,6 +27,8 @@
 RTC_INIT_HISTOGRAM = 'Media.RTCVideoDecoderInitDecodeSuccess'
 MEDIA_GVD_BUCKET = 0
 RTC_VIDEO_INIT_BUCKET = 1
+MEDIA_AGVDA_INIT_STATUS = 'Media.ArcGpuVideoDecodeAccelerator.InitializeResult'
+MEDIA_AGVDA_BUCKET = 0
 
 #Path for video HTML file which helps for video operations
 CROS_VIDEO_DIR = '/usr/local/autotest/cros/video'