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 b38abad373b6044c22287436cbbee1a4db543a4c..abf4ea888d5b313b1f5768ab2f245dfb86e75045 100644
--- a/chrome/browser/chrome_browser_field_trials.cc
+++ b/chrome/browser/chrome_browser_field_trials.cc
@@ -7,7 +7,10 @@
 #include <string>
 
 #include "base/command_line.h"
+#include "base/feature_list.h"
 #include "base/metrics/field_trial.h"
+#include "base/metrics/histogram_base.h"
+#include "base/metrics/histogram_persistence.h"
 #include "base/strings/string_util.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
@@ -21,6 +24,24 @@
 #include "chrome/browser/chrome_browser_field_trials_desktop.h"
 #endif
 
+namespace {
+
+void InstantiatePersistentHistograms() {

[Alexei Svitkine (slow), 2016/01/22 16:15:16]

Nit: Add a comment.

[bcwhite, 2016/01/22 17:17:48]

Done.

+  if (base::FeatureList::IsEnabled(base::kPersistentHistogramsFeature)) {
+    const std::string allocator_name("BrowserMetricsAllocator");
+    // Create persistent/shared memory and allow histograms to be stored in it.
+    // TODO(bcwhite): Update this with correct allocator and memory size.
+    base::SetPersistentHistogramMemoryAllocator(
+        new base::LocalPersistentMemoryAllocator(1 << 20,     // 1 MiB
+                                                 0x4D5B9953,  // SHA1(B..M..A..)
+                                                 allocator_name));
+    base::GetPersistentHistogramMemoryAllocator()->CreateTrackingHistograms(
+        allocator_name);
+  }
+}
+
+}  // namespace
+
 ChromeBrowserFieldTrials::ChromeBrowserFieldTrials(
     const base::CommandLine& parsed_command_line)
     : parsed_command_line_(parsed_command_line) {
@@ -41,6 +62,8 @@ void ChromeBrowserFieldTrials::SetupFieldTrials() {
 }
 
 void ChromeBrowserFieldTrials::InstantiateDynamicTrials() {
+  // Persistent histograms must be enabled as soon as possible.
+  InstantiatePersistentHistograms();
   // The following trials are used from renderer process.
   // Mark here so they will be sync-ed.
   base::FieldTrialList::FindValue("DisplayList2dCanvas");