service worker: Remove unused strings for navigation preload flag.

The ServiceWorkerNavigationPreload feature flag was removed in r494256.

Bug: 753230
Change-Id: Ie2fe09c477dbffe1bc19cc050f1e6f413b1a8358
Reviewed-on: https://chromium-review.googlesource.com/1148021Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577447}

chrome/browser/flag_descriptions.cc

@@ -1553,12 +1553,6 @@ const char kSecondaryUiMdDescription[] =
     "browser dialogs. If --top-chrome-md is effectively Material Design "
     "refresh, this option is enabled.";
 
-const char kServiceWorkerNavigationPreloadName[] =
-    "Service worker navigation preload.";
-const char kServiceWorkerNavigationPreloadDescription[] =
-    "Enable web pages to use the experimental service worker navigation "
-    "preload API.";
-
 const char kServiceWorkerPaymentAppsName[] = "Service Worker payment apps";
 const char kServiceWorkerPaymentAppsDescription[] =
     "Enable Service Worker applications to integrate as payment apps";

chrome/browser/flag_descriptions.h

@@ -950,9 +950,6 @@ extern const char kScrollPredictionDescription[];
 extern const char kSecondaryUiMd[];
 extern const char kSecondaryUiMdDescription[];
 
-extern const char kServiceWorkerNavigationPreloadName[];
-extern const char kServiceWorkerNavigationPreloadDescription[];
-
 extern const char kServiceWorkerPaymentAppsName[];
 extern const char kServiceWorkerPaymentAppsDescription[];