media/gpu/test/VEClient: Fix a hang issue of --disable_validator

This fixes an issue that FlushDone() in VideoEncoderClient will
never be called when the test runs with --disable_validator and
the VideoEncodeAccelerator doesn't support Flush().

Bug: None
Test: video_encode_acclerator_tests --disable_validator on trogdor
Change-Id: I8701a971deaa4cacfcd55a2c708de437a79ad863
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537507Reviewed-by: David Staessens <dstaessens@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828005}

media/gpu/test/video_encoder/video_encoder_client.cc

@@ -366,14 +366,13 @@ void VideoEncoderClient::BitstreamBufferReady(
 
   if (bitstream_processors_.empty()) {
     BitstreamBufferProcessed(bitstream_buffer_id);
-    return;
-  }
-
+  } else {
     auto bitstream_ref = CreateBitstreamRef(bitstream_buffer_id, metadata);
     ASSERT_TRUE(bitstream_ref);
     for (auto& bitstream_processor_ : bitstream_processors_) {
       bitstream_processor_->ProcessBitstream(bitstream_ref, frame_index_);
     }
+  }
   frame_index_++;
   FlushDoneTaskIfNeeded();
 }