Implement ServiceWorkerFetchDispatcher::MaybeStartNavigationPreload().

content/browser/service_worker/service_worker_fetch_dispatcher.cc

Index: content/browser/service_worker/service_worker_fetch_dispatcher.cc
diff --git a/content/browser/service_worker/service_worker_fetch_dispatcher.cc b/content/browser/service_worker/service_worker_fetch_dispatcher.cc
index fefc7137f6e953a3a34b43563943dc094bab16ab..453bc93f6ff2e476d3b2691edd1bbb8274de3bcf 100644
--- a/content/browser/service_worker/service_worker_fetch_dispatcher.cc
+++ b/content/browser/service_worker/service_worker_fetch_dispatcher.cc
@@ -12,6 +12,7 @@
 #include "base/time/time.h"
 #include "base/trace_event/trace_event.h"
 #include "content/browser/service_worker/embedded_worker_status.h"
+#include "content/browser/service_worker/service_worker_navigation_preload_fetcher.h"
 #include "content/browser/service_worker/service_worker_version.h"
 #include "content/common/service_worker/fetch_event_dispatcher.mojom.h"
 #include "content/common/service_worker/service_worker_messages.h"
@@ -279,9 +280,20 @@ void ServiceWorkerFetchDispatcher::MaybeStartNavigationPreload(
   }
   if (!version_->navigation_preload_enabled())
     return;
-  // TODO(horo): Implement this to start the preload request for the navigation
-  // request and set |preload_handle_|.
-  NOTIMPLEMENTED();
+  // TODO(horo): Currently NavigationPreload doesn't support request body.
+  if (!request_->blob_uuid.empty())
+    return;
+  if (!base::CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kEnableExperimentalWebPlatformFeatures)) {
+    // TODO(horo): Check |version_|'s origin_trial_tokens() here if we use
+    // Origin-Trial for NavigationPreload.
+    return;
+  }
+  // TODO(horo): Pass "Service-Worker-Navigation-Preload" header to
+  // ServiceWorkerNavigationPreloadFetcher.
+  // See: https://github.com/w3c/ServiceWorker/issues/920#issuecomment-251150270
+  preload_handle_ =
+      ServiceWorkerNavigationPreloadFetcher::Start(original_request);
 }
 
 ServiceWorkerMetrics::EventType ServiceWorkerFetchDispatcher::GetEventType()