Move GetFetchClientSettingsObject() to FetchContext

As preparation for plumbing FetchClientSettingsObject via
FetchContext rather than as an argument,
this CL exposes GetFetchClientSettingsObject() in FetchContext.

NullFetchContext is added just to make
FetchContext::GetFetchClientSettingsObject() abstract and
FetchContext::NullInstance()'s GetFetchClientSettingsObject()
non-null.

This CL doesn't change the behavior.

Bug: 880027
Change-Id: I02a428d77d202ab88cffd22a3fdf3cd34eff4aca
Reviewed-on: https://chromium-review.googlesource.com/c/1200042
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606759}
4 files changed