Implement ServiceWorkerFetchDispatcher::MaybeStartNavigationPreload().

This CL depends on https://codereview.chromium.org/2417793002.

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/ This CL.
[3/4] Implement FetchEvent.navigationPreload.
      https://codereview.chromium.org/2416843002/
[4/4] Add browser tests for NavigationPreload.
      https://codereview.chromium.org/2413643005/

BUG=649558

Review-Url: https://chromiumcodereview.appspot.com/2410333006
Cr-Commit-Position: refs/heads/master@{#426423}
19 files changed