ServiceWorker: Keep the worker alive until FetchEvent.waitUntil settles

content/browser/service_worker/service_worker_metrics.h

Index: content/browser/service_worker/service_worker_metrics.h
diff --git a/content/browser/service_worker/service_worker_metrics.h b/content/browser/service_worker/service_worker_metrics.h
index aca36c9e351e3bebeceacdef260a0b22e1c7df25..3732bd25d7caa514319f010ddb73b13328a2fd7b 100644
--- a/content/browser/service_worker/service_worker_metrics.h
+++ b/content/browser/service_worker/service_worker_metrics.h
@@ -95,6 +95,7 @@ class ServiceWorkerMetrics {
     FETCH_SUB_RESOURCE = 13,
     UNKNOWN = 14,  // Used when event type is not known.
     FOREIGN_FETCH = 15,
+    FETCH_WAITUNTIL = 16,

[falken, 2016/06/09 19:28:48]

It's unfortunate that WAITUNTIL is not an "event" since this is an EventType.
Should this be RequestType? I'm not sure the naming is making sense.

[shimazu, 2016/06/10 10:26:49]

Yes, I know. Hmm, I don't have good name now. 
It's mostly used as "EventType". EventType is currently used for
StartRequest(WithCustomTimer) and RecordStartWorkerResult.
RecordStartWorker is called in StartWorker, RunAfterStartWorker,
DidEnsureLiveRegistrationForStartWorker, and I fell it's a bit odd for
StartWorker to get a RequestType because StartWorker itself doesn't request a
message sent back from the service worker.

[falken, 2016/06/16 06:23:09]

I don't think it's so odd, since StartWorker itself doesn't dispatch an event
either. But this enum is also not really exactly "request" since it also splits
up fetch into MAIN_FRAME, SUB_FRAME, etc. for UMA purposes.

Anyway, I think we can go ahead and just put a comment explaining what EventType
is and how it's mostly events but also requests, since events typically have a
1:1 correspondence with requests except for the fetch event.

[shimazu, 2016/06/21 02:43:37]

Done.

     // Add new events to record here.
     NUM_TYPES
   };