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/media/base/mediachannel.h b/media/base/mediachannel.h
index c1cc156..c7d8580 100644
--- a/media/base/mediachannel.h
+++ b/media/base/mediachannel.h
@@ -88,7 +88,7 @@
     ost << vals[i].ToString();
   }
   ost << "]";
-  return ost.str();
+  return ost.Release();
 }
 
 // Options that can be applied to a VideoMediaChannel or a VideoMediaEngine.
@@ -120,7 +120,7 @@
                          screencast_min_bitrate_kbps);
     ost << ToStringIfSet("is_screencast ", is_screencast);
     ost << "}";
-    return ost.str();
+    return ost.Release();
   }
 
   // Enable denoising? This flag comes from the getUserMedia
@@ -157,7 +157,7 @@
     ost << "uri: " << uri;
     ost << ", id: " << id;
     ost << "}";
-    return ost.str();
+    return ost.Release();
   }
 
   std::string uri;
@@ -626,7 +626,7 @@
       separator = ", ";
     }
     ost << "}";
-    return ost.str();
+    return ost.Release();
   }
 
  protected: