Add histograms for data usage

chrome/browser/android/data_usage/external_data_use_observer_bridge.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/android/data_usage/external_data_use_observer_bridge.h"
 
 #include <vector>
 
 #include "base/android/context_utils.h"
 #include "base/android/jni_string.h"
@@ skipping 101 matching lines @@
   }
 
   DCHECK_EQ(app_package_name_native.size(), domain_path_regex_native.size());
   DCHECK_EQ(app_package_name_native.size(), label_native.size());
 
   if (!data_use_tab_model_)
     return;
 
   data_use_tab_model_->RegisterURLRegexes(
       app_package_name_native, domain_path_regex_native, label_native);
+
+  io_task_runner_->PostTask(
+      FROM_HERE, base::Bind(&ExternalDataUseObserver::OnFetchMatchingRulesDone,

[tbansal1, 2016/01/13 18:06:52]

What are we trying to measure here? The way it is instrumented now, it will also
take into account time taken for 2 thread hops, which is not useful.

May be the purpose is to measure the time it takes since the start of Chrome to
until the matching rules are first fetched (because all bytes during that time
would be lost -- and we want to know that)?

I guess after the first fetch, it is not important how much time the subsequent
fetches take (e.g., even if they take 1 minute, we are still okay because rules
are valid for much longer duration).

[Raj, 2016/01/13 23:33:51]

Agreed. I guess duration for first fetch is more valuable, than for subsequent
fetches.
But I thought the thread hops are negligible, and adding up those times don't
matter? 

+                            external_data_use_observer_));
 }
 
 void ExternalDataUseObserverBridge::ReportDataUse(
     const std::string& label,
     net::NetworkChangeNotifier::ConnectionType connection_type,
     const std::string& mcc_mnc,
     const base::Time& start_time,
     const base::Time& end_time,
     int64_t bytes_downloaded,
     int64_t bytes_uploaded) const {
@@ skipping 32 matching lines @@
   data_use_tab_model_->OnControlAppInstalled();
 }
 
 bool RegisterExternalDataUseObserver(JNIEnv* env) {
   return RegisterNativesImpl(env);
 }
 
 }  // namespace android
 
 }  // namespace chrome