Store field trial parameters in shared memory

base/metrics/field_trial.h

Index: base/metrics/field_trial.h
diff --git a/base/metrics/field_trial.h b/base/metrics/field_trial.h
index 509ace4f77df5a6ec62a80d13eb18f19813a482b..14793bf5606f97e411633213234ad0b0b328e8d9 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -69,6 +69,7 @@
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/shared_memory.h"
+#include "base/metrics/field_trial_param_associator.h"
 #include "base/metrics/persistent_memory_allocator.h"
 #include "base/observer_list_threadsafe.h"
 #include "base/process/launch.h"
@@ -428,6 +429,9 @@ class BASE_EXPORT FieldTrialList {
   // metrics, crashes, etc.
   static std::string FindFullName(const std::string& trial_name);
 
+  static bool FindParams(const std::string& trial_name,
+                         FieldTrialParamAssociator::FieldTrialParams* params);

[Alexei Svitkine (slow), 2016/11/02 14:56:46]

I worry about this API being public. It may mislead people into thinking they
could use it, when it's meant to be internal.

For example, someone may expect this to work from the browser process. Can you
come up with a design that keeps it private?

One thought I had was if we make it private but have a function in
FieldTrialParamAssociator() to register a callback to that function, which can
be done e.g. during the creation of the first FieldTrialList.

Now, that approach will need to have the SetCallback function public - but a)
that function is less likely to be mistaken by users for something useful and b)
we can enforce that it can only be used once.

... Now, the other approach could be to merge the two classes - FieldTrialList
and FieldTrialParamAssociator - but that possibly seems like a bigger change?
Well, at least splitting the files would be since you need to update all the
includes. Another option is to not split the files and just merge it in. I guess
I'm open to the idea, but I'd have to see the CL.

However, maybe to not expand the scope of this CL too much, go with the callback
approach first for now?

+
   // Returns true if the named trial has been registered.
   static bool TrialExists(const std::string& trial_name);
 
@@ -539,6 +543,7 @@ class BASE_EXPORT FieldTrialList {
   // Allow tests to access our innards for testing purposes.
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, InstantiateAllocator);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, AddTrialsToAllocator);
+  FRIEND_TEST_ALL_PREFIXES(FieldTrialListTest, AssociateFieldTrialParams);
 
   // Expects a mapped piece of shared memory |shm| that was created from the
   // browser process's field_trial_allocator and shared via the command line.