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}

modules/video_coding/codecs/test/videocodec_test_fixture_impl.cc

@@ -279,7 +279,7 @@ std::string VideoCodecTestFixtureImpl::Config::ToString() const {
     }
   }
   ss << "\n";
-  return ss.str();
+  return ss.Release();
 }
 
 std::string VideoCodecTestFixtureImpl::Config::CodecName() const {