Create a synthetic field trial for precache

chrome/browser/precache/precache_util.cc

Index: chrome/browser/precache/precache_util.cc
diff --git a/chrome/browser/precache/precache_util.cc b/chrome/browser/precache/precache_util.cc
index 049155ba5122cea054071d7eb1b8884612c5a18d..7d4ae419ab3320faaf27e48767989ccf5f704fc8 100644
--- a/chrome/browser/precache/precache_util.cc
+++ b/chrome/browser/precache/precache_util.cc
@@ -4,8 +4,10 @@
 
 #include "chrome/browser/precache/precache_util.h"
 
-#include "base/time/time.h"
+#include "base/metrics/field_trial.h"
+#include "base/strings/string_util.h"
 #include "chrome/browser/browser_process.h"
+#include "chrome/browser/metrics/chrome_metrics_service_accessor.h"
 #include "chrome/browser/precache/precache_manager_factory.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_manager.h"
@@ -21,6 +23,13 @@ class HttpResponseInfo;
 
 namespace {
 
+const char kPrecacheFieldTrialName[] = "Precache";
+const char kPrecacheSyntheticFieldTrialName[] = "PrecacheSynthetic";
+
+// Users who precached in the last |kDaysForPrecacheCandidates| days will be
+// treated as precache candidates.
+const size_t kDaysForPrecacheCandidates = 15;
+
 void UpdatePrecacheMetricsAndStateOnUIThread(const GURL& url,
                                              const GURL& referrer,
                                              base::TimeDelta latency,
@@ -70,4 +79,16 @@ void UpdatePrecacheMetricsAndState(const net::URLRequest* request,
                  data_use_measurement::IsUserRequest(*request), profile_id));
 }
 
+void RegisterPrecacheSyntheticFieldTrial(const base::Time& last_precache_time) {
+  if (base::Time::Now() - last_precache_time <=
+      base::TimeDelta::FromDays(kDaysForPrecacheCandidates)) {
+    const auto& group_name =
+        base::FieldTrialList::FindFullName(kPrecacheFieldTrialName);
+    if (!group_name.empty()) {
+      ChromeMetricsServiceAccessor::RegisterSyntheticFieldTrial(

[twifkak, 2016/12/08 21:49:22]

Ooh, how about a multi-group field trial? Just learned about it now. It occurs
to me the next question we will ask is "What about only people who precached
yesterday?"

Some proof-of-concept code:

const size_t kDaysForRecentPrecacheCandidates = 1;

std::vector<uint32_t> groups;
base::TimeDelta time_ago = base::Time::Now() - last_precache_time;
if (time_ago <= base::TimeDelta::FromDays(kDaysForPrecacheCandidates))
  groups.push_back(metrics::HashName(group_name);
if (time_ago <=
    base::TimeDelta::FromDays(kDaysForRecentPrecacheCandidates))
  groups.push_back(metrics::HashName(group_name + "Recent");
RegisterSyntheticMultiGroupFieldTrial(
  kPrecacheSyntheticFieldTrialName, groups);

+          kPrecacheSyntheticFieldTrialName, group_name);
+    }
+  }
+}
+
 }  // namespace precache