Use CoreProbeSink instead of ExecutionContext in FetchContext

To prepare for further decoupling outsideSettings WorkerFetchContext,
this CL reduces dependency to ExecutionContext from FetchContext
by switching them to CoreProbeSink.

Bug: 880027
Change-Id: I9cc8739178d04e553f16aa977b3805a13f873c5e
Reviewed-on: https://chromium-review.googlesource.com/c/1193382
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618114}
9 files changed