Support experiment to store persistent metrics in memory-mapped file.

chrome/browser/metrics/chrome_metrics_service_client.cc

Index: chrome/browser/metrics/chrome_metrics_service_client.cc
diff --git a/chrome/browser/metrics/chrome_metrics_service_client.cc b/chrome/browser/metrics/chrome_metrics_service_client.cc
index 2861db7cdd25856894100c062c4ab3cc835428f0..3f0bd7d2efe89d38effad86da9d89041f1cbac33 100644
--- a/chrome/browser/metrics/chrome_metrics_service_client.cc
+++ b/chrome/browser/metrics/chrome_metrics_service_client.cc
@@ -213,6 +213,20 @@ ChromeMetricsServiceClient::ChromeMetricsServiceClient(
 
 ChromeMetricsServiceClient::~ChromeMetricsServiceClient() {
   DCHECK(thread_checker_.CalledOnValidThread());
+
+  // If there is a global metrics file being updated on disk, mark it to be

[Alexei Svitkine (slow), 2016/06/16 11:36:57]

Please make a function in the anon namespace that does this and just call it
from here.

By the way, are you sure this object is guaranteed to be deleted?

[bcwhite, 2016/06/16 14:21:08]

Done.

I checked that the object is deleted on clean shutdown.  I don't know if it's
guaranteed but it's not a problem if it is not and probably desired to keep it
around it if anything abnormal happens.

+  // deleted when the process exits. A normal shutdown is almost complete
+  // so there is no benefit in keeping a file with no new data to be processed
+  // during the next startup sequence.
+  base::GlobalHistogramAllocator* allocator =
+      base::GlobalHistogramAllocator::Get();
+  if (allocator) {
+    base::FilePath path = allocator->GetPersistentLocation();
+    if (!path.empty()) {
+      base::File file(path, base::File::FLAG_OPEN | base::File::FLAG_READ |
+                            base::File::FLAG_DELETE_ON_CLOSE);
+    }
+  }
 }
 
 // static