Make --force-fieldtrials not activate them in the browser process.

base/metrics/field_trial.h

Index: base/metrics/field_trial.h
===================================================================
--- base/metrics/field_trial.h	(revision 208942)
+++ base/metrics/field_trial.h	(working copy)
@@ -392,8 +392,10 @@
   // used in a non-browser process, to carry randomly selected state in a
   // browser process into this non-browser process, but could also be invoked
   // through a command line argument to the browser process. The created field
-  // trials are marked as "used" for the purposes of active trial reporting.
-  static bool CreateTrialsFromString(const std::string& prior_trials);
+  // trials are marked as "used" for the purposes of active trial reporting if
+  // |activate_trials| is true.
+  static bool CreateTrialsFromString(const std::string& prior_trials,
+                                     bool activate_trials);

[Ilya Sherman, 2013/06/28 00:58:07]

nit: Please define an enumerated constant for this, rather than passing in a
boolean, which is hard to interpret at the call sites.  Alternately, since there
seems to be only one client of this code that actually wants to activate all of
the field trials, maybe it makes sense to move the activation logic out of this
method?

[Alexei Svitkine (slow), 2013/06/28 17:05:15]

Done.

 
   // Create a FieldTrial with the given |name| and using 100% probability for
   // the FieldTrial, force FieldTrial to have the same group string as