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/api/candidate.cc b/api/candidate.cc
index 128c470..c2c6c53 100644
--- a/api/candidate.cc
+++ b/api/candidate.cc
@@ -78,7 +78,7 @@
       << ":" << related_address_.ToString() << ":" << username_ << ":"
       << password_ << ":" << network_id_ << ":" << network_cost_ << ":"
       << generation_ << "]";
-  return ost.str();
+  return ost.Release();
 }
 
 uint32_t Candidate::GetPriority(uint32_t type_preference,