Worker: Consolidate setup routines of WebWorkerFetchContextImpl

This just cleans up setup routines of WebWorkerFetchContextImpl, and doesn't
change functional behavior.

Bug: 906991
Change-Id: I01cb433b31399c36964adbda9cb5a1c799ec3677
Reviewed-on: https://chromium-review.googlesource.com/c/1412096
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623145}
4 files changed