ServiceWorker: Avoid creating a new SWNetworkProvider when navigation happens within a page
ServiceWorker: Avoid creating a new SWNetworkProvider when navigation happens within a page

Before this patch, navigation within a page (eg. location.hash, pushState())
unexpectedly disables the fetch event routing on Service Worker because such
navigation always creates a new ServiceWorkerNetworkProvider instance even
though actual page transition does not occur.

This change avoids creating a new provider on the navigation-within-page and
keeps the initial service worker even if the document url changed into either
an out-of-scope url or an in-scope url controlled by other service worker.

Layout tests will be added by https://codereview.chromium.org/758413002/

BUG=429972,433708
TEST=run-webkit-tests http/tests/serviceworker (https://codereview.chromium.org/758413002/)

Review URL: https://codereview.chromium.org/744533002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#306017}

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