Move task runner from FetchContext to ResourceFetcher

As part of FetchContext cleanup, this CL moves the loading task runner
from FetchContext::GetLoadingTaskRunner() to
ResourceFetcher::GetTaskRunner(), because "for loading" part is
apparent, and there is no other task runners in ResourceFetcher.

Bug: 914739
Change-Id: I1e0af9ddac1ce35b95bd0deb13e0bfc2572b7a0f
Reviewed-on: https://chromium-review.googlesource.com/c/1411713
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624033}
27 files changed