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

content/browser/browser_main_runner.cc

Index: content/browser/browser_main_runner.cc
diff --git a/content/browser/browser_main_runner.cc b/content/browser/browser_main_runner.cc
index 96a3454f1499edbb0211afbcdb1ba8024cb6e2bc..6bbb4d2777d2862e3a0b679377e5ccc1ba3a22d3 100644
--- a/content/browser/browser_main_runner.cc
+++ b/content/browser/browser_main_runner.cc
@@ -11,6 +11,7 @@
 #include "base/macros.h"
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_macros.h"
+#include "base/metrics/persistent_histogram_allocator.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/profiler/scoped_profile.h"
 #include "base/profiler/scoped_tracker.h"
@@ -60,6 +61,15 @@ class BrowserMainRunnerImpl : public BrowserMainRunner {
   }
 
   int Initialize(const MainFunctionParams& parameters) override {
+    // Create persistent/shared memory and allow histograms to be stored in it.
+    // Memory that is not actualy used won't be physically mapped by the system.
+    // BrowserMetrics usage peaked around 1.9MiB as of 2016-02-20.

[Ilya Sherman, 2016/05/04 06:48:08]

When you say that it peaked around 1.9MiB, do you mean on a test run on your
machine, or globally across all Chrome users?

[bcwhite, 2016/05/04 14:12:51]

As reported in UMA across all (enrolled) users.

+    base::GlobalHistogramAllocator::CreateWithLocalMemory(
+        3 << 20,     // 3 MiB
+        0x935DDD43,  // SHA1(BrowserMetrics)
+        "BrowserMetrics");
+    base::GlobalHistogramAllocator::Disable();  // Enabled by experiment only.
+
     SCOPED_UMA_HISTOGRAM_LONG_TIMER(
         "Startup.BrowserMainRunnerImplInitializeLongTime");