media/gpu/v4l2: use NOTREACHED when V4L2Buffer::GetVideoFrame() is called improperly

V4L2Buffer::GetVideoFrame() is an internal method that is never exposed
outside of V4L2Device. Change a regular error handling (which would be
the way to do if this was exposed) into a NOTREACHED() so we catch
potential bugs in our code.

BUG=None
TEST=VDAtest --use_vd passes on Kukui.

Change-Id: Ide5cf00447df154d39d1400d156633ea67b45e2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2143426
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Jeffrey Kardatzke <jkardatzke@google.com>
Cr-Commit-Position: refs/heads/master@{#759137}

media/gpu/v4l2/v4l2_device.cc

@@ -240,9 +240,9 @@ scoped_refptr<VideoFrame> V4L2Buffer::GetVideoFrame() {
   // We can create the VideoFrame only when using MMAP buffers.
   if (v4l2_buffer_.memory != V4L2_MEMORY_MMAP) {
     VLOGF(1) << "Cannot create video frame from non-MMAP buffer";
-    // video_frame_ should be null since that's its default value.
-    DCHECK_EQ(video_frame_, nullptr);
-    return video_frame_;
+    // Allow NOTREACHED() on invalid argument because this is an internal
+    // method.
+    NOTREACHED();
   }
 
   // Create the video frame instance if requiring it for the first time.