Set default max_threads_count to DVQA injection helper

Bug: webrtc:12247
Change-Id: I608cffad7ee5397c306fb03a36d89f31882c112c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/196092
Commit-Queue: Andrey Logvin <landrey@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32748}

test/pc/e2e/analyzer/video/video_quality_analyzer_injection_helper.h

@@ -75,7 +75,7 @@ class VideoQualityAnalyzerInjectionHelper : public StatsObserverInterface {
 
   void Start(std::string test_case_name,
              rtc::ArrayView<const std::string> peer_names,
-             int max_threads_count);
+             int max_threads_count = 1);
 
   // Forwards |stats_reports| for Peer Connection |pc_label| to
   // |analyzer_|.