Set up field trial for precache experiment

chrome/browser/chrome_browser_field_trials_mobile.cc

Index: chrome/browser/chrome_browser_field_trials_mobile.cc
diff --git a/chrome/browser/chrome_browser_field_trials_mobile.cc b/chrome/browser/chrome_browser_field_trials_mobile.cc
index 500d560fb2d6a307576115bc0852a8c1dc3219f8..8d522f84491841388cac3262e3911a80fea3c120 100644
--- a/chrome/browser/chrome_browser_field_trials_mobile.cc
+++ b/chrome/browser/chrome_browser_field_trials_mobile.cc
@@ -14,6 +14,9 @@
 
 namespace chrome {
 
+const char kEnabled[] = "Enabled";
+const char kDisabled[] = "Disabled";
+
 namespace {
 
 // Base function used by all data reduction proxy field trials. A trial is
@@ -25,8 +28,6 @@ void DataCompressionProxyBaseFieldTrial(
     const char* trial_name,
     const base::FieldTrial::Probability enabled_group_probability,
     const base::FieldTrial::Probability total_probability) {
-  const char kEnabled[] = "Enabled";
-  const char kDisabled[] = "Disabled";
 
   // Find out if this is a stable channel.
   const bool kIsStableChannel =
@@ -66,12 +67,29 @@ void DataCompressionProxyFieldTrials() {
       "DataCompressionProxyPromoVisibility", 100, 1000);
 }
 
+// Configures the "Precache" field trial. A trial is only conducted for installs
+// that are in the "Enabled" group. Until the percentage is learned from the
+// server, a client-side configuration is used.
+void PrecacheFieldTrial() {

[Alexei Svitkine (slow), 2014/02/12 15:41:50]

I'm not sure this block of code actually buys you anything, since this will be a
server-driven experiment.

Basically, it's still possible to have a server-driven experiment without this
code being present. The only thing this buys is a configuration when the Finch
config isn't available (e.g. first run after the initial install). But since
this hardcoded config sets it to 100% disabled, that shouldn't be any different
than not having a config.

+  // Experiment enabled until December 31, 2015. By default, disabled.
+  scoped_refptr<base::FieldTrial> trial(
+      base::FieldTrialList::FactoryGetFieldTrial(
+          "Precache", 1000, kDisabled, 2015, 12, 31,
+          base::FieldTrial::ONE_TIME_RANDOMIZED, NULL));
+
+  const int kEnabledGroup = trial->AppendGroup(kEnabled, 0);
+
+  VLOG(1) << "Precache enabled group id: " << kEnabledGroup
+          << ". Selected group id: " << trial->group();
+}
+
 }  // namespace
 
 void SetupMobileFieldTrials(const CommandLine& parsed_command_line,
                             const base::Time& install_time,
                             PrefService* local_state) {
   DataCompressionProxyFieldTrials();
+  PrecacheFieldTrial();
 }
 
 }  // namespace chrome