Introduce chrome://flags/#network-prediction-options-for-service-worker

content/public/browser/service_worker_context.h

Index: content/public/browser/service_worker_context.h
diff --git a/content/public/browser/service_worker_context.h b/content/public/browser/service_worker_context.h
index 22d13f9bd57ac84e19382a9430fc1a88f20b0199..e0a4074988f9f1594ce5068c249e1095200874fa 100644
--- a/content/public/browser/service_worker_context.h
+++ b/content/public/browser/service_worker_context.h
@@ -14,6 +14,8 @@
 
 namespace content {
 
+class RenderProcessHost;
+
 enum class ServiceWorkerCapability {
   NO_SERVICE_WORKER,
   SERVICE_WORKER_NO_FETCH_HANDLER,
@@ -139,6 +141,16 @@ class ServiceWorkerContext {
   // be called on the UI thread.
   virtual void ClearAllServiceWorkersForTest(const base::Closure& callback) = 0;
 
+  // Starts a Service Worker for |document_url| for a navigation preconnect in
+  // the specified render process |host|. Must be called from the UI thread. The

[falken, 2017/06/01 06:54:17]

The "for... for... in..." is ambiguous: is the document in host, or preconnect
in host?

This function itself also doesn't really have to do NavigationPreconnect. It
just starts the worker and the caller happens to be the predictor, and it's used
even if preconnect won't happen. I wonder if just StartServiceWorker is a better
name?

I'd suggest:

"Starts the service worker whose scope matches |document_url|. The service
worker is started in |host|. This can be used to warm up a service worker for a
navigation."

[horo, 2017/06/01 11:15:07]

I changed it to "StartServiceWorkerForNavigation".
I think we should keep "Navigation" for the method name, because UMAs of
EventType::NAVIGATION will be recorded if this method is called.

[falken, 2017/06/01 23:24:14]

Sorry I totally overlooked the EventType. I think PRECONNECT was a better name
than NAVIGATION for the event type.

I'm concerned we're creating in this separate path for starting a service worker
for navigation different than the normal navigation path (starting a service
worker via URLRequestJob), just to start before the predictor. If we do create
this separate path the name PRECONNECT feels better to make it explicit that
it's a separate path.

It's worth noting that this CL will also affect our metrics about navigations
and timing like:
ServiceWorker.ActivatedWorkerPreparationForMainFrame.

+  // |callback| will always be called on the UI thread. This method can fail if:
+  //  * No Service Worker was registered for |document_url|.
+  //  * The specified render process is not suitable for loading |document_url|.
+  virtual void StartServiceWorkerForNavigationPreconnect(
+      const GURL& document_url,
+      RenderProcessHost* host,
+      const ResultCallback& callback) = 0;
+
  protected:
   ServiceWorkerContext() {}
   virtual ~ServiceWorkerContext() {}