Add a feature guard to implement CrOS VA-API JPEG decode acceleration.

This CL creates a base::Feature flag in content to guard the
implementation of decode acceleration of JPEG images (as opposed to
camera captures) in Chrome OS using the VA-API.

Bug: 868400
Change-Id: I103a78bc9547862620fea59fb423582226f03d59
Reviewed-on: https://chromium-review.googlesource.com/1180292
Commit-Queue: Andres Calderon Jaramillo <[email protected]>
Reviewed-by: Avi Drissman <[email protected]>
Reviewed-by: Miguel Casas <[email protected]>
Cr-Commit-Position: refs/heads/master@{#584606}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index e245ad3..b92597a 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -2115,6 +2115,9 @@
 extern const char kUseMonitorColorSpaceName[];
 extern const char kUseMonitorColorSpaceDescription[];
 
+extern const char kVaapiJpegImageDecodeAccelerationName[];
+extern const char kVaapiJpegImageDecodeAccelerationDescription[];
+
 extern const char kVideoPlayerChromecastSupportName[];
 extern const char kVideoPlayerChromecastSupportDescription[];