[DVQA] Sum the encoded_image_size for simulcast scenario.

This aligns actual_encode_bitrate with target_encode_bitrate

Change-Id: I8c460a5883e5eacbee8e67b5b6625f65792efb77
Bug: b/340790240
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/350620
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Jeremy Leconte <jleconte@google.com>
Cr-Commit-Position: refs/heads/main@{#42310}

test/pc/e2e/analyzer/video/default_video_quality_analyzer_frame_in_flight.cc

diff --git a/test/pc/e2e/analyzer/video/default_video_quality_analyzer_frame_in_flight.cc b/test/pc/e2e/analyzer/video/default_video_quality_analyzer_frame_in_flight.cc
index 056bd73..c2851f5 100644
--- a/test/pc/e2e/analyzer/video/default_video_quality_analyzer_frame_in_flight.cc
+++ b/test/pc/e2e/analyzer/video/default_video_quality_analyzer_frame_in_flight.cc
@@ -93,7 +93,7 @@
         *time_between_encoded_frames;
   }
   frame_type_ = frame_type;
-  encoded_image_size_ = encoded_image_size;
+  encoded_image_size_ += encoded_image_size;
   target_encode_bitrate_ += target_encode_bitrate;
   stream_layers_qp_[stream_index].AddSample(SamplesStatsCounter::StatsSample{
       .value = static_cast<double>(qp), .time = time});