[CrOS MultiDevice] Improve logging performance.

Previously std::stringstream::str() was called twice, which generates
two distinct (but identical) strings. This CL caches the result of the
call and uses it twice.

Bug: 862725
Change-Id: I0384981a259b12d6c3ecf1420436f71e1e002fe6
Reviewed-on: https://chromium-review.googlesource.com/1134218Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574625}

chromeos/components/proximity_auth/logging/logging.cc

@@ -30,13 +30,14 @@ ScopedLogMessage::~ScopedLogMessage() {
   if (!g_logging_enabled)
     return;
 
+  const std::string string_from_stream = stream_.str();
   LogBuffer::GetInstance()->AddLogMessage(LogBuffer::LogMessage(
-      stream_.str(), base::Time::Now(), file_, line_, severity_));
+      string_from_stream, base::Time::Now(), file_, line_, severity_));
 
   // The destructor of |log_message| also creates a log for the standard logging
   // system.
   logging::LogMessage log_message(file_, line_, severity_);
-  log_message.stream() << stream_.str();
+  log_message.stream() << string_from_stream;
 }
 
 }  // namespace proximity_auth