Measure the X-Chrome-Variations construction time with UMA.

chrome/browser/metrics/variations/variations_http_header_provider.cc

Index: chrome/browser/metrics/variations/variations_http_header_provider.cc
diff --git a/chrome/browser/metrics/variations/variations_http_header_provider.cc b/chrome/browser/metrics/variations/variations_http_header_provider.cc
index 1e7d7877cf3242a34d1e5fbf2e958076538b2728..bcc7a4a4d1cf2d7a3d58efc68b0d8541f9720e9f 100644
--- a/chrome/browser/metrics/variations/variations_http_header_provider.cc
+++ b/chrome/browser/metrics/variations/variations_http_header_provider.cc
@@ -6,6 +6,7 @@
 
 #include "base/base64.h"
 #include "base/memory/singleton.h"
+#include "base/metrics/histogram.h"
 #include "chrome/browser/google/google_util.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_io_data.h"
@@ -92,6 +93,8 @@ void VariationsHttpHeaderProvider::InitVariationIDsCacheIfNeeded() {
   DCHECK(MessageLoop::current());
   base::FieldTrialList::AddObserver(this);
 
+  base::TimeTicks before_time = base::TimeTicks::Now();
+
   base::FieldTrial::ActiveGroups initial_groups;
   base::FieldTrialList::GetActiveFieldTrialGroups(&initial_groups);
   for (base::FieldTrial::ActiveGroups::const_iterator it =
@@ -105,6 +108,13 @@ void VariationsHttpHeaderProvider::InitVariationIDsCacheIfNeeded() {
   }
   UpdateVariationIDsHeaderValue();
 
+  UMA_HISTOGRAM_CUSTOM_COUNTS(
+      "Variations.HeaderConstructionTime",
+      (base::TimeTicks::Now() - before_time).InMicroseconds(),
+      0,
+      base::TimeDelta::FromSeconds(1).InMicroseconds(),
+      50);
+
   variation_ids_cache_initialized_ = true;
 }