Fix hardware encoding for cast mirroring on CrOS

Fix a bug where a scoped_refptr was incorrectly scoped in an if-block,
causing encoding to fail.

Bug: 991287
Change-Id: Iebce34a1349c968bc808ac5a225ae33df6845c73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1748079Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686603}

media/cast/sender/external_video_encoder.cc

@@ -208,7 +208,7 @@ class ExternalVideoEncoder::VEAClientImpl
           input_buffers_[index].get();
       DCHECK(input_buffer->first.IsValid());
       DCHECK(input_buffer->second.IsValid());
-      scoped_refptr<media::VideoFrame> frame = VideoFrame::WrapExternalData(
+      frame = VideoFrame::WrapExternalData(
           video_frame->format(), frame_coded_size_, video_frame->visible_rect(),
           video_frame->visible_rect().size(),
           input_buffer->second.GetMemoryAsSpan<uint8_t>().data(),