Plumb a WebWorkerFetchContext for nested workers

Bug: 829119
Change-Id: Ie393ac9e18bdfcded983d98a16c00254b0954547
Reviewed-on: https://chromium-review.googlesource.com/1000519
Commit-Queue: Nate Chapin <japhet@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551391}
19 files changed