media/gpu/vaapi: cleanup: use sizeof(variable) ISO sizeof(type)

crrev.com/c/2401512 suggested using sizeof(variable) ISO sizeof(type)
for various reasons [1]. This CL follows up by cleaning a previously
landed similar idiom.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2401512/comment/8675d0dc_90c8dccc/

Bug: b/166646505
Change-Id: I86f21fd4e12f8c1b4402ec2520391f138a402f5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2403680
Auto-Submit: Miguel Casas <mcasas@chromium.org>
Commit-Queue: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: Jeffrey Kardatzke <jkardatzke@google.com>
Cr-Commit-Position: refs/heads/master@{#805818}

media/gpu/vaapi/vp9_vaapi_video_decoder_delegate.cc

@@ -139,10 +139,8 @@ bool VP9VaapiVideoDecoderDelegate::SubmitDecode(
   }
 
   if (!vaapi_wrapper_->SubmitBuffers(
-          {{VAPictureParameterBufferType,
-            sizeof(VADecPictureParameterBufferVP9), &pic_param},
-           {VASliceParameterBufferType, sizeof(VASliceParameterBufferVP9),
-            &slice_param},
+          {{VAPictureParameterBufferType, sizeof(pic_param), &pic_param},
+           {VASliceParameterBufferType, sizeof(slice_param), &slice_param},
            {VASliceDataBufferType, frame_hdr->frame_size, frame_hdr->data}})) {
     return false;
   }