Use a different algorithm with the low entropy source for field trials.

chrome/browser/metrics/metrics_service.h

Index: chrome/browser/metrics/metrics_service.h
===================================================================
--- chrome/browser/metrics/metrics_service.h	(revision 152081)
+++ chrome/browser/metrics/metrics_service.h	(working copy)
@@ -16,6 +16,7 @@
 #include "base/gtest_prod_util.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
+#include "base/metrics/field_trial.h"
 #include "base/process_util.h"
 #include "chrome/browser/metrics/metrics_log.h"
 #include "chrome/browser/metrics/tracking_synchronizer_observer.h"
@@ -86,16 +87,21 @@
   // recording is not currently running.
   std::string GetClientId();
 
-  // Returns the preferred entropy source used to seed persistent activities
+  // Returns the preferred entropy provider used to seed persistent activities
   // based on whether or not metrics reporting will permitted on this client.

[jar (doing other things), 2012/08/17 19:06:19]

nit: typo:   "metrics reporting will permitted"

[Alexei Svitkine (slow), 2012/08/20 15:57:40]

Done.

   // The caller must determine if metrics reporting will be enabled for this
-  // client and pass that state in as |reporting_will_be_enabled|. If
-  // |reporting_will_be_enabled| is true, this method returns the client ID
-  // concatenated with the low entropy source. Otherwise, this method just
-  // returns the low entropy source. Note that this reporting state can not be
-  // checked by reporting_active() because this method may need to be called
-  // before the MetricsService needs to be started.
-  std::string GetEntropySource(bool reporting_will_be_enabled);
+  // client and pass that state in as |reporting_will_be_enabled|.
+  //
+  // If |reporting_will_be_enabled| is true, this method returns an entropy
+  // provider that has a high source of entropy, partially based on the client
+  // ID. Otherwise, an entropy provider that is based on a low entropy source
+  // is returned.
+  //
+  // Note that this reporting state can not be checked by reporting_active()
+  // because this method may need to be called before the MetricsService needs
+  // to be started.
+  scoped_ptr<base::FieldTrial::EntropyProvider> GetEntropyProvider(

[jar (doing other things), 2012/08/17 19:06:19]

nit: I'd prefer to see an out-parameter for the entropy provider, as this
pattern caused me to have to think far too much about what is going on.  

At a minimum, it would help to have a name that makes it clear that a malloc
(new) is being done. A "getter" traditinoally leaves some ownership with the
instance. Perhaps:
void NewEntropyProvider(bool reporting_will_be_enabled,
                        const EntropyProvider* entropy_provider);

or (kindred to your scoped_ptr pattern):

void NewEntropyProvider(bool reporting_will_be_enabled,
                        scoped_ptr<const EntropyProvider>* entropy_provider);

Also, why is a mutable EntropyProvide() needed, rather than a const instance? 

[Ilya Sherman, 2012/08/17 19:14:00]

FWIW, I think we used to use out params for things like this, but are now
gradually moving over to returning a scoped_ptr<> directly, now that we have the
Pass() method available to support this generally.

[Alexei Svitkine (slow), 2012/08/20 15:57:40]

Changed name to CreateEntropyProvider() and made the returned provider const.
Still kept it returning a scoped_ptr per the same reasoning as Ilya mentioned.

+      bool reporting_will_be_enabled);
 
   // Force the client ID to be generated. This is useful in case it's needed
   // before recording.