Make fewer copies when using StringBuilder.

Replace calls to .str() which copies with .Release which moves in cases where that's safe.

This CL was generated by this command:
git grep -l 'StringBuilder' |
xargs perl -i -0 -pe "s/(rtc::StringBuilder (\S+);.*?return )\\g2.str\(\)/\$1\$2.Release\(\)/sg"

Bug: webrtc:8982
Change-Id: If4dadbeb039df010aaaa9e58da81c1971a84fe8f
Reviewed-on: https://webrtc-review.googlesource.com/100307
Commit-Queue: Jonas Olsson <jonasolsson@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24790}
diff --git a/video/video_quality_test.cc b/video/video_quality_test.cc
index 9596cfa..7f7c873 100644
--- a/video/video_quality_test.cc
+++ b/video/video_quality_test.cc
@@ -250,7 +250,7 @@
   if (params_.ss[0].num_spatial_layers > 1)
     ss << ", Layer #" << params_.ss[0].selected_sl;
   ss << ")";
-  return ss.str();
+  return ss.Release();
 }
 
 void VideoQualityTest::CheckParamsAndInjectionComponents() {