[OfflinePages] Adds TimeToStart UMA for background load requests.

BUG=656099

Review-Url: https://codereview.chromium.org/2440733002
Cr-Commit-Position: refs/heads/master@{#427186}

components/offline_pages/background/request_coordinator.cc

@@ -74,6 +74,23 @@ void RecordOfflinerResultUMA(const ClientId& client_id,
   }
 }
 
+void RecordStartTimeUMA(const SavePageRequest& request) {
+  std::string histogram_name("OfflinePages.Background.TimeToStart");
+  if (base::SysInfo::IsLowEndDevice()) {
+    histogram_name += ".Svelte";
+  }
+
+  // The histogram below is an expansion of the UMA_HISTOGRAM_CUSTOM_TIMES
+  // macro adapted to allow for a dynamically suffixed histogram name.
+  // Note: The factory creates and owns the histogram.
+  base::HistogramBase* histogram = base::Histogram::FactoryTimeGet(
+      AddHistogramSuffix(request.client_id(), histogram_name.c_str()),
+      base::TimeDelta::FromMilliseconds(100), base::TimeDelta::FromDays(7), 50,
+      base::HistogramBase::kUmaTargetedHistogramFlag);
+  base::TimeDelta duration = base::Time::Now() - request.creation_time();
+  histogram->AddTime(duration);
+}
+
 void RecordCancelTimeUMA(const SavePageRequest& canceled_request) {
   // Using regular histogram (with dynamic suffix) rather than time-oriented
   // one to record samples in seconds rather than milliseconds.
@@ -581,6 +598,11 @@ void RequestCoordinator::SendRequestToOffliner(const SavePageRequest& request) {
   DCHECK(!is_busy_);
   is_busy_ = true;
 
+  // Record start time if this is first attempt.
+  if (request.started_attempt_count() == 0) {
+    RecordStartTimeUMA(request);
+  }
+
   // Mark attempt started in the database and start offliner when completed.
   queue_->MarkAttemptStarted(
       request.request_id(),

tools/metrics/histograms/histograms.xml

@@ -39107,6 +39107,22 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="OfflinePages.Background.TimeToStart" units="ms">
+  <owner>dougarnett@chromium.org</owner>
+  <summary>
+    Time from background request until processing it was first started. This is
+    for non-svelte devices.
+  </summary>
+</histogram>
+
+<histogram name="OfflinePages.Background.TimeToStart.Svelte" units="ms">
+  <owner>dougarnett@chromium.org</owner>
+  <summary>
+    Time from background request until processing it was first started on svelte
+    device. Svelte is determined by base::SysInfo::IsLowEnd().
+  </summary>
+</histogram>
+
 <histogram name="OfflinePages.Background.UnsupportedScheme.ConnectionType"
     enum="NetworkConnectionType">
   <owner>dougarnett@chromium.org</owner>
@@ -108675,6 +108691,8 @@ value.
   <affected-histogram name="OfflinePages.Background.OfflinerRequestStatus"/>
   <affected-histogram name="OfflinePages.Background.TimeToCanceled"/>
   <affected-histogram name="OfflinePages.Background.TimeToSaved"/>
+  <affected-histogram name="OfflinePages.Background.TimeToStart"/>
+  <affected-histogram name="OfflinePages.Background.TimeToStart.Svelte"/>
   <affected-histogram name="OfflinePages.DeletePage.AccessCount"/>
   <affected-histogram name="OfflinePages.DeletePage.LastOpenToCreated"/>
   <affected-histogram name="OfflinePages.DeletePage.PageSize"/>