Add async tracing to SWVersion::AddRequest.

content/browser/service_worker/service_worker_metrics.cc

Index: content/browser/service_worker/service_worker_metrics.cc
diff --git a/content/browser/service_worker/service_worker_metrics.cc b/content/browser/service_worker/service_worker_metrics.cc
index 64c8d4325b58e5483b8b3e63b48433bde7fdabfe..2df2b135893506899293006f01035b698b2deffd 100644
--- a/content/browser/service_worker/service_worker_metrics.cc
+++ b/content/browser/service_worker/service_worker_metrics.cc
@@ -44,8 +44,28 @@ enum EventHandledRatioType {
   NUM_EVENT_HANDLED_RATIO_TYPE,
 };
 
+const char* const kEventTypeNames[] = {"Activate",
+                                       "Install",
+                                       "Fetch",
+                                       "Sync",
+                                       "Notification Click",
+                                       "Push",
+                                       "Geofencing",
+                                       "Service Port Connect",
+                                       "Message"};
+
+static_assert(arraysize(kEventTypeNames) ==
+                  static_cast<int>(ServiceWorkerMetrics::EventType::NUM_TYPES),
+              "Names should match event types");
+
 }  // namespace
 
+const char* ServiceWorkerMetrics::EventTypeToString(EventType event_type) {
+  size_t event_type_as_int = static_cast<size_t>(event_type);
+  DCHECK(event_type_as_int < arraysize(kEventTypeNames));
+  return kEventTypeNames[event_type_as_int];

[nhiroki, 2016/01/25 03:50:23]

switch-case could be more self-contained. That can check whether all enum types
are appropriately handled on compile time like static_assert.

switch (event_type) {
  case EventType::ACTIVATE:
    return "Activate";
  case EventType::INSTALL:
    return "Install";
  ...
}
NOTREACHED() << event_type;
return "Unknown";

[Marijn Kruisselbrink, 2016/01/25 19:17:29]

Done

+}
+
 bool ServiceWorkerMetrics::ShouldExcludeSiteFromHistogram(Site site) {
   return site == ServiceWorkerMetrics::Site::NEW_TAB_PAGE;
 }