Support experiment configurations with/without stability metrics.

chrome/browser/chrome_browser_field_trials.cc

Index: chrome/browser/chrome_browser_field_trials.cc
diff --git a/chrome/browser/chrome_browser_field_trials.cc b/chrome/browser/chrome_browser_field_trials.cc
index 816b9222d7ddf8c5acbf3a472a25ff7123f41577..b1a30e42be6190d19b6a30b98ec6dd0b250f0248 100644
--- a/chrome/browser/chrome_browser_field_trials.cc
+++ b/chrome/browser/chrome_browser_field_trials.cc
@@ -23,6 +23,7 @@
 #include "chrome/common/channel_info.h"
 #include "chrome/common/chrome_paths.h"
 #include "chrome/common/chrome_switches.h"
+#include "components/metrics/file_metrics_provider.h"
 #include "components/metrics/metrics_pref_names.h"
 #include "components/variations/variations_associated_data.h"
 
@@ -80,6 +81,12 @@ void InstantiatePersistentHistograms() {
         active_file, kAllocSize, kAllocId,
         ChromeMetricsServiceClient::kBrowserMetricsName);
     result = MAPPED_FILE_SUCCESS;
+    // Some configurations don't return unreported metrics from the previous
+    // run (otherwise known as "stability metrics").
+    std::string unreported = variations::GetVariationParamValueByFeature(
+        base::kPersistentHistogramsFeature, "send_unreported_metrics");
+    metrics::FileMetricsProvider::SetStabilityMetricsEnabled(
+        unreported == "yes");

[Alexei Svitkine (slow), 2016/11/24 21:27:58]

This would change the meaning of the existing groups.

How about making the new param be in the negative (e.g.
disable_stability_uploads or something)?

[bcwhite, 2016/11/25 18:51:57]

I've never liked the double negatives that occur that way but there's an easy
fix:  != "no".

   } else if (storage == "LocalMemory") {
     // Use local memory for storage even though it will not persist across
     // an unclean shutdown.