Support saving browser metrics to disk and reading them during next run.

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 648167695f6b70b9ca8b5c986e7c773d7d08c2ca..cc913f874073ab51febfacd1b2ee9f82e80d8287 100644
--- a/chrome/browser/chrome_browser_field_trials.cc
+++ b/chrome/browser/chrome_browser_field_trials.cc
@@ -8,12 +8,15 @@
 
 #include "base/command_line.h"
 #include "base/feature_list.h"
+#include "base/files/file_path.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/persistent_histogram_allocator.h"
+#include "base/path_service.h"
 #include "base/strings/string_util.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
+#include "chrome/common/chrome_paths.h"
 #include "chrome/common/chrome_switches.h"
 #include "components/metrics/metrics_pref_names.h"
 
@@ -39,6 +42,20 @@ void InstantiatePersistentHistograms() {
         kAllocatorName);
     base::GlobalHistogramAllocator::Get()->CreateTrackingHistograms(
         kAllocatorName);
+
+    // Set the destination for where these will be persistent. This shouldn't

[Ilya Sherman, 2016/04/19 00:57:46]

nit: s/persistent/persisted

[bcwhite, 2016/04/19 16:33:38]

Done.

+    // be necessary because it's done in chrome_metrics_service_client.cc where
+    // the reading of this file is configured but field trials don't get done
+    // until after that point so there is no global allocator to configure
+    // when that code runs. This won't be a problem when this (eventually)
+    // becomes standard and is enabled during early startup.
+    base::FilePath metrics_file;
+    if (base::PathService::Get(chrome::DIR_USER_DATA, &metrics_file)) {
+      metrics_file = metrics_file.AppendASCII(kAllocatorName)
+                         .AddExtension(FILE_PATH_LITERAL(".pma"));
+      base::GlobalHistogramAllocator::Get()
+          ->SetPersistentLocation(metrics_file);
+    }

[Ilya Sherman, 2016/04/19 00:57:46]

I think I'd prefer to always set the location from the correct location, and to
just call a boolean Enable() method based on field trial state.  WDYT?

[bcwhite, 2016/04/19 16:33:38]

I could add Disable/Enable methods to the GlobalHistogramAllocator class.  I'll
look into it.

   }
 }