ServiceWorker: Move WebServiceWorkerNetworkProviderImplForFrame into its own file

This just moves the class declaration and definition for cleanup. There're no
functional changes.

Bug: n/a
Change-Id: Ie58b7d6c91bf6356fcf1fae2e7105e9d56fabffb
Reviewed-on: https://chromium-review.googlesource.com/c/1424681
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624532}
4 files changed