Introduce ResourceFetcher::LoaderFactory

Introduce ResourceFetcher::LoaderFactory to migrate
FetchContext::CreateURLLoader and FetchContext::CreateCodeCacheLoader.
Move the implementations from FetchContext subclasses to LoaderFactory
subclasses. Remove the FetchContext::GetLoadingTaskRunner as the moved
functions were the last users.

Bug: 914739
Change-Id: Id887bff75652ea39cc93d231b32ec2a749be2f87
Reviewed-on: https://chromium-review.googlesource.com/c/1414744
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627239}
26 files changed