Make PrintResultMeanAndError receive two doubles instead of a string.

Bug: webrtc:8566
Change-Id: Ida925b030bff24275d34c0e888ee362e94c46b21
Reviewed-on: https://webrtc-review.googlesource.com/25540
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Reviewed-by: Alex Loiko <aleloi@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Commit-Queue: Edward Lemur <ehmaldonado@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20866}

test/testsupport/perf_test.h

@@ -47,7 +47,8 @@ void PrintResult(const std::string& measurement,
 void PrintResultMeanAndError(const std::string& measurement,
                              const std::string& modifier,
                              const std::string& trace,
-                             const std::string& mean_and_error,
+                             const double mean,
+                             const double error,
                              const std::string& units,
                              bool important);