Add browser tests for NavigationPreload.

This CL depends on https://codereview.chromium.org/2416843002/

FetchEvent.navigationPreload will be implemented in the following steps.
[1/4] Introduce FetchEventPreloadHandle to pass the preload response to FetchEvent.
      https://codereview.chromium.org/2417793002/
[2/4] Implement ServiceWorkerFetchDispatcher::MaybeStartNavigationPreload().
      https://codereview.chromium.org/2410333006/
[3/4] Implement FetchEvent.navigationPreload.
      https://codereview.chromium.org/2416843002/
[4/4] Add browser tests for NavigationPreload.
      https://codereview.chromium.org/2413643005/ This CL.

Currently NavigationPreloadManager.enable() API is not implemented yet.
So in those tests, we sets this flag by directly calling ServiceWorkerVersion.
set_navigation_preload_enabled()

BUG=649558
R=falken@chromium.org

Review URL: https://codereview.chromium.org/2413643005 .

Cr-Commit-Position: refs/heads/master@{#427705}
2 files changed