Introduce Origin-Trial for Service Worker Navigation Preload

content/browser/service_worker/service_worker_version.h

Index: content/browser/service_worker/service_worker_version.h
diff --git a/content/browser/service_worker/service_worker_version.h b/content/browser/service_worker/service_worker_version.h
index 57ac26103f94e93dbb8a8c7dc900aa3961bce276..701e08adb06ad5756c52665fca512ca69f38ae92 100644
--- a/content/browser/service_worker/service_worker_version.h
+++ b/content/browser/service_worker/service_worker_version.h
@@ -427,6 +427,36 @@ class CONTENT_EXPORT ServiceWorkerVersion
   void NotifyMainScriptJobCreated(
       ServiceWorkerContextRequestHandler::CreateJobStatus status);
 
+  // Returns true if the service worker should support Navigation Preload.
+  // There are three possible states:
+  //  - A: Navigation Preload related methods and attributes are available in JS
+  //       and work correctly.
+  //  - B: Navigation Preload related methods and attributes are available in
+  //       JS. But NavigationPreloadManager's enable, disable and setHeaderValue
+  //       methods always return a rejected promise. And FetchEvent's
+  //       preloadResponse attribute returns a promise which always resolve with
+  //       undefined.
+  //  - C: Navigation Preload related methods and attributes are not available
+  //       in JS.
+  // This method returns true only for A case.
+  // blink::OriginTrials::serviceWorkerNavigationPreloadEnabled() returns true
+  // for both A and B case.
+  //
+  //  * Origin Trial: Have an effective header.
+  //                                 Command line
+  //                             Default  Enable  Disabled
+  //                   Default      B       A        B
+  //      Field trial  Enabled      A       A        B
+  //                   Disabled     B       A        B
+  //
+  //  * Origin Trial: No header.
+  //                                 Command line
+  //                             Default  Enable  Disabled
+  //                   Default      C       A        C
+  //      Field trial  Enabled      C       A        C
+  //                   Disabled     C       A        C

[falken, 2017/01/13 03:43:41]

Great comment.

+  bool IsNavigationPreloadSupported() const;
+
  private:
   friend class base::RefCounted<ServiceWorkerVersion>;
   friend class ServiceWorkerMetrics;