[OfflinePages] Adds TimeToStart UMA for background load requests.

components/offline_pages/background/request_coordinator.cc

Index: components/offline_pages/background/request_coordinator.cc
diff --git a/components/offline_pages/background/request_coordinator.cc b/components/offline_pages/background/request_coordinator.cc
index d1d380b005043d4a541073c48f6832a005653476..290ae155f2e08ee700e3d5f0dcc40ef1aa798de2 100644
--- a/components/offline_pages/background/request_coordinator.cc
+++ b/components/offline_pages/background/request_coordinator.cc
@@ -74,6 +74,19 @@ 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";
+  }
+
+  base::TimeDelta duration = base::Time::Now() - request.creation_time();
+  UMA_HISTOGRAM_CUSTOM_TIMES(

[Steven Holte, 2016/10/20 20:51:11]

Also you shouldn't use this macro with a computed histogram name, since it uses
a static pointer to the histogram.  Instead, use
base::Histogram::FactoryTimeGet() (or split this into seperate macro invokations
for each histogram name)

[dougarnett, 2016/10/21 16:13:48]

Done. Thanks

+      AddHistogramSuffix(request.client_id(), histogram_name.c_str()), duration,
+      base::TimeDelta::FromMilliseconds(100),
+      base::TimeDelta::FromSeconds(kMaxDurationSeconds), kDurationBuckets);

[Steven Holte, 2016/10/20 20:46:31]

You don't want this bound to change even if you change this constant for some
other reason (it will make buckets incompatible between new and old versions of
chrome), so it would be better to just inline the value here.

[dougarnett, 2016/10/21 16:13:48]

Done.

+}
+
 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.
@@ -547,6 +560,11 @@ 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(),