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

content/common/gpu/media/dxva_video_decode_accelerator_win.cc

Index: content/common/gpu/media/dxva_video_decode_accelerator_win.cc
diff --git a/content/common/gpu/media/dxva_video_decode_accelerator_win.cc b/content/common/gpu/media/dxva_video_decode_accelerator_win.cc
index b4ee8f0b682e59b2309eac1898e05a2b30f5146c..2f270108550b78b543d306c1a134cadc7bd902f8 100644
--- a/content/common/gpu/media/dxva_video_decode_accelerator_win.cc
+++ b/content/common/gpu/media/dxva_video_decode_accelerator_win.cc
@@ -940,6 +940,11 @@ bool DXVAVideoDecodeAccelerator::Initialize(const Config& config,
     return false;
   }
 
+  if (config.output_mode != Config::OutputMode::ALLOCATE) {
+    NOTREACHED() << "Only ALLOCATE OutputMode is supported by this VDA";
+    return false;
+  }
+
   client_ = client;
 
   main_thread_task_runner_ = base::MessageLoop::current()->task_runner();