VAAPI-JDA: OutputPicture should fail if I420 conversion fails.

This CL makes VaapiJpegDecodeAccelerator::OutputPicture() return early
if it's not possible to convert the image to I420 format. Otherwise, it
would be wrongly notifying the client of success in this case.

Bug: 924295
Test: JDA unit tests pass on an eve.
Change-Id: I57f558e2e0061ceef93600a9c373596150035fa5
Reviewed-on: https://chromium-review.googlesource.com/c/1427880Reviewed-by: Miguel Casas <mcasas@chromium.org>
Commit-Queue: Andres Calderon Jaramillo <andrescj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624962}

media/gpu/vaapi/vaapi_jpeg_decode_accelerator.cc

@@ -414,6 +414,7 @@ bool VaapiJpegDecodeAccelerator::OutputPicture(
     default:
       VLOGF(1) << "Can't convert image to I420: unsupported format 0x"
                << std::hex << va_image_format.fourcc;
+      return false;
   }
 
   task_runner_->PostTask(