Remove helper GetForBrowserContext() from PrefetchServiceFactory.

PrefetchServiceFactory is now a SimpleKeyedServiceFactory, so its
callers should provide a SimpleFactoryKey to access it.

The helper method was added as a transitional state, but is not
necessary in the long run.

Bug: 953918
Change-Id: Ib5aa1c3323d486be27c372114bae1483bfedc59c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599745
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661857}
8 files changed