ServiceWorker: Remove WebRuntimeFeatures::EnableServiceWorkerNavigationPreload()

NavigationPreload is enabled by default[1] and the function is no longer used.

[1] https://www.chromestatus.com/feature/5734842339688448

Bug: 661071
Change-Id: I36955d9458743dc38fda0c31f6900c728b772f94
Reviewed-on: https://chromium-review.googlesource.com/989522Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548000}

third_party/WebKit/public/platform/WebRuntimeFeatures.h

@@ -129,7 +129,6 @@ class WebRuntimeFeatures {
   BLINK_PLATFORM_EXPORT static void EnableScriptedSpeech(bool);
   BLINK_PLATFORM_EXPORT static void EnableScrollAnchoring(bool);
   BLINK_PLATFORM_EXPORT static void EnableScrollAnchorSerialization(bool);
-  BLINK_PLATFORM_EXPORT static void EnableServiceWorkerNavigationPreload(bool);
   BLINK_PLATFORM_EXPORT static void EnableServiceWorkerScriptFullCodeCache(
       bool);
   BLINK_PLATFORM_EXPORT static void EnableSharedArrayBuffer(bool);