Support external buffer import in VDA interface and add a V4L2SVDA impl.

content/common/gpu/media/android_video_decode_accelerator.cc

Index: content/common/gpu/media/android_video_decode_accelerator.cc
diff --git a/content/common/gpu/media/android_video_decode_accelerator.cc b/content/common/gpu/media/android_video_decode_accelerator.cc
index 13b54ee5e654a9789fd96d8959b4ef5410da2fee..b189024ff7b28a7d16ef24532d92c1f6114e5a03 100644
--- a/content/common/gpu/media/android_video_decode_accelerator.cc
+++ b/content/common/gpu/media/android_video_decode_accelerator.cc
@@ -294,6 +294,11 @@ bool AndroidVideoDecodeAccelerator::Initialize(const Config& config,
     return false;
   }
 
+  if (config.output_mode != Config::OutputMode::ALLOCATE) {
+    NOTREACHED() << "Only ALLOCATE OutputMode is supported by this VDA";
+    return false;
+  }
+
   DCHECK(client);
   client_ = client;
   codec_ = VideoCodecProfileToVideoCodec(config.profile);