Let PrintResultMeanAndError accept a string.

Some downstream projects still use it. I'll update them and then revert
this change.

Bug: webrtc:8566
Change-Id: Ib4e56348c40a3645f3049382b47089ca6c675e96
Reviewed-on: https://webrtc-review.googlesource.com/25841
Commit-Queue: Edward Lemur <ehmaldonado@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20872}

test/testsupport/perf_test.h

@@ -52,6 +52,13 @@ void PrintResultMeanAndError(const std::string& measurement,
                              const std::string& units,
                              bool important);
 
+void PrintResultMeanAndError(const std::string& measurement,
+                             const std::string& modifier,
+                             const std::string& trace,
+                             const std::string& mean_and_error,
+                             const std::string& units,
+                             bool important);
+
 // Like PrintResult(), but prints an entire list of results. The |values|
 // will generally be a list of comma-separated numbers. A typical
 // post-processing step might produce plots of their mean and standard