Add histograms for data usage

chrome/browser/android/data_usage/external_data_use_observer_bridge.cc

Index: chrome/browser/android/data_usage/external_data_use_observer_bridge.cc
diff --git a/chrome/browser/android/data_usage/external_data_use_observer_bridge.cc b/chrome/browser/android/data_usage/external_data_use_observer_bridge.cc
index 71ad96c18be6d3cd2446dc2be470b42e046d6195..6fbf32c2efe6788a55131d9eac8f82d1dea7bb62 100644
--- a/chrome/browser/android/data_usage/external_data_use_observer_bridge.cc
+++ b/chrome/browser/android/data_usage/external_data_use_observer_bridge.cc
@@ -10,6 +10,7 @@
 #include "base/android/jni_string.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/field_trial.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/single_thread_task_runner.h"
 #include "base/time/time.h"
 #include "chrome/browser/android/data_usage/data_use_tab_model.h"
@@ -81,8 +82,10 @@ void ExternalDataUseObserverBridge::Init(
   FetchMatchingRules();
 }
 
-void ExternalDataUseObserverBridge::FetchMatchingRules() const {
+void ExternalDataUseObserverBridge::FetchMatchingRules() {
   DCHECK(thread_checker_.CalledOnValidThread());
+  if (matching_rules_first_fetch_time_.is_null())

[tbansal1, 2016/01/14 06:59:55]

make it a const variable, and set it in the construtor? Then, we will know how
much time it takes from ~start of chrome to first fetch?

[Raj, 2016/01/15 06:23:24]

Done.

+    matching_rules_first_fetch_time_ = base::TimeTicks::Now();
 
   JNIEnv* env = base::android::AttachCurrentThread();
   Java_ExternalDataUseObserver_fetchMatchingRules(
@@ -96,6 +99,7 @@ void ExternalDataUseObserverBridge::FetchMatchingRulesDone(
     const base::android::JavaParamRef<jobjectArray>& domain_path_regex,
     const base::android::JavaParamRef<jobjectArray>& label) {
   DCHECK(thread_checker_.CalledOnValidThread());
+  DCHECK(!matching_rules_first_fetch_time_.is_null());
 
   // Convert to native objects.
   std::vector<std::string> app_package_name_native;
@@ -117,6 +121,10 @@ void ExternalDataUseObserverBridge::FetchMatchingRulesDone(
   if (!data_use_tab_model_)
     return;
 
+  UMA_HISTOGRAM_TIMES(
+      "DataUsage.Perf.MatchingRuleFetchDuration",
+      base::TimeTicks::Now() - matching_rules_first_fetch_time_);

[tbansal1, 2016/01/14 06:59:55]

How do we know this is the first fetch? Seems like this will get recorded every
15 minutes.

[Raj, 2016/01/15 06:23:24]

Fixed

+
   data_use_tab_model_->RegisterURLRegexes(
       app_package_name_native, domain_path_regex_native, label_native);
 }