Support "shared" histograms between processes.

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 9ce113f9675862b6cb790df76d78bb9d4ac5e5a8..8f5416921acae98e4794a781edf78d2f600bd1a4 100644
--- a/chrome/browser/chrome_browser_field_trials.cc
+++ b/chrome/browser/chrome_browser_field_trials.cc
@@ -8,6 +8,7 @@
 
 #include "base/command_line.h"
 #include "base/metrics/field_trial.h"
+#include "base/metrics/histogram_base.h"
 #include "base/strings/string_util.h"
 #include "base/time/time.h"
 #include "chrome/common/chrome_switches.h"
@@ -40,6 +41,19 @@ void ChromeBrowserFieldTrials::SetupFieldTrials() {
 }
 
 void ChromeBrowserFieldTrials::InstantiateDynamicTrials() {
+  // Persistent histograms must be enabled as soon as possible.
+  const std::string group_name =
+      base::FieldTrialList::FindFullName("PersistentMetrics");
+  if (base::StartsWith(
+          group_name, "Enabled", base::CompareCase::INSENSITIVE_ASCII) &&
+      !parsed_command_line_.HasSwitch(switches::kDisablePersistentMetrics)) {

[Alexei Svitkine (slow), 2015/12/15 22:48:52]

Please use the base/feature_list.h API instead. This way, you don't need to
check both the field trial and the command line, just the feature.

[bcwhite, 2015/12/16 16:23:46]

Done.

+    // Create persistent/shared memory and allow histograms to be stored in it.
+    // TODO(bcwhite): Update this with correct allocator and memory size.
+    base::HistogramBase::SetDefaultPersistentMemoryAllocator(
+        new base::LocalPersistentMemoryAllocator(1 << 20,     // 1 MiB
+                                                 0x4D5B9953,  // SHA1(B..M..A..)
+                                                 "BrowserMetricsAllocator"));
+  }
   // The following trials are used from renderer process.
   // Mark here so they will be sync-ed.
   base::FieldTrialList::FindValue("DisplayList2dCanvas");