Remove webrtc::test::InitFieldTrialsFromString(const std::string&).

This is done to solve a problem where a string literal is implicitly cast
to a temporary std::string when calling webrtc::test::InitFieldTrialsFromString
which passes a pointer to the internal representation to
webrtc::field_trial::InitFieldTrialFromString(char*). This pointer is
stored for later use, but the temporary std::string is destroyed as soon
as the function returns.

Using webrtc::field_trial::InitFieldTrialFromString(char*) instead,
avoids the implicit casts (but the caller still needs to ensure that
the char* outlives the program). The validation previously done by
webrtc::test::InitFieldTrialsFromString can now be done by manually
calling webrtc::test::ValidateFieldTrialsStringOrDie(const std::string&).

Add system_wrappers:field_trial_default as a direct dependency to
various targets to allow including the field_trials_default.h header.

Bug: webrtc:8812
Change-Id: Ib5a641ea255b1c16a8f7f35e1fe67f6c38a61da6
Reviewed-on: https://webrtc-review.googlesource.com/46141
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Oleh Prypin <oprypin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21856}

rtc_tools/event_log_visualizer/main.cc

@@ -15,6 +15,7 @@
 #include "rtc_tools/event_log_visualizer/analyzer.h"
 #include "rtc_tools/event_log_visualizer/plot_base.h"
 #include "rtc_tools/event_log_visualizer/plot_python.h"
+#include "system_wrappers/include/field_trial_default.h"
 #include "test/field_trial.h"
 #include "test/testsupport/fileutils.h"
 
@@ -195,7 +196,10 @@ int main(int argc, char* argv[]) {
   }
 
   webrtc::test::SetExecutablePath(argv[0]);
-  webrtc::test::InitFieldTrialsFromString(FLAG_force_fieldtrials);
+  webrtc::test::ValidateFieldTrialsStringOrDie(FLAG_force_fieldtrials);
+  // InitFieldTrialsFromString stores the char*, so the char array must outlive
+  // the application.
+  webrtc::field_trial::InitFieldTrialsFromString(FLAG_force_fieldtrials);
 
   std::string filename = argv[1];