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}

rtc_base/strings/string_builder.h

@@ -157,8 +157,8 @@ class StringBuilder {
   size_t size() const { return str_.size(); }
 
   std::string Release() {
-    std::string ret;
-    std::swap(str_, ret);
+    std::string ret = std::move(str_);
+    str_.clear();
     return ret;
   }