Adding Profile.Incognito.Lifetime metric for iOS.

Profile.Incognito.Lifetime metric is not working in iOS currently. This
CL add Profile.Incognito.Lifetime metric to iOS app.

Bug: 1067163
Change-Id: I17cdc2fe493a15c8a1cd57bbf6abb2d7429bab11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2137344
Commit-Queue: Side YILMAZ <sideyilmaz@chromium.org>
Reviewed-by: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757458}

ios/chrome/browser/browser_state/off_the_record_chrome_browser_state_impl.cc

@@ -5,6 +5,7 @@
 #include "ios/chrome/browser/browser_state/off_the_record_chrome_browser_state_impl.h"
 
 #include "base/logging.h"
+#include "base/metrics/histogram_functions.h"
 #include "base/sequenced_task_runner.h"
 #include "base/task/post_task.h"
 #include "components/keyed_service/ios/browser_state_dependency_manager.h"
@@ -25,6 +26,7 @@ OffTheRecordChromeBrowserStateImpl::OffTheRecordChromeBrowserStateImpl(
     : ChromeBrowserState(std::move(io_task_runner)),
       otr_state_path_(otr_path),
       original_chrome_browser_state_(original_chrome_browser_state),
+      start_time_(base::Time::Now()),
       prefs_(CreateIncognitoBrowserStatePrefs(
           static_cast<sync_preferences::PrefServiceSyncable*>(
               original_chrome_browser_state->GetPrefs()))) {
@@ -40,6 +42,11 @@ OffTheRecordChromeBrowserStateImpl::~OffTheRecordChromeBrowserStateImpl() {
   if (pref_proxy_config_tracker_)
     pref_proxy_config_tracker_->DetachFromPrefService();
 
+  const base::TimeDelta duration = base::Time::Now() - start_time_;
+  base::UmaHistogramCustomCounts(
+      "Profile.Incognito.Lifetime", duration.InMinutes(), 1,
+      base::TimeDelta::FromDays(28).InMinutes(), 100);
+
   // Clears any data the network stack contains that may be related to the
   // OTR session.
   GetApplicationContext()->GetIOSChromeIOThread()->ChangedToOnTheRecord();

ios/chrome/browser/browser_state/off_the_record_chrome_browser_state_impl.h

@@ -52,6 +52,9 @@ class OffTheRecordChromeBrowserStateImpl : public ChromeBrowserState {
   base::FilePath otr_state_path_;
   ChromeBrowserState* original_chrome_browser_state_;  // weak
 
+  // Creation time of the off-the-record BrowserState.
+  const base::Time start_time_;
+
   std::unique_ptr<sync_preferences::PrefServiceSyncable> prefs_;
 
   std::unique_ptr<OffTheRecordChromeBrowserStateIOData::Handle> io_data_;