gpu_arc_vea: Remove completion callback after invocation.

The callback is used for only once. There is no need to keep it in
the callback map.

Bug: None
Test: None
Change-Id: I0a40faa323b1c9f98ad05f15bcb8451fe28041c2
Reviewed-on: https://chromium-review.googlesource.com/683915
Commit-Queue: Kuang-che Wu <kcwu@chromium.org>
Reviewed-by: Kuang-che Wu <kcwu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504321}

chrome/gpu/gpu_arc_video_encode_accelerator.cc

@@ -63,6 +63,7 @@ void GpuArcVideoEncodeAccelerator::BitstreamBufferReady(
   auto iter = use_bitstream_cbs_.find(bitstream_buffer_id);
   DCHECK(iter != use_bitstream_cbs_.end());
   iter->second.Run(payload_size, key_frame, timestamp.InMicroseconds());
+  use_bitstream_cbs_.erase(iter);
 }
 
 void GpuArcVideoEncodeAccelerator::NotifyError(Error error) {