Split cache: Fill network isolation key for dedicated workers.

We send the network isolation key to WorkerScriptFetchInitiator
::Start.  This change impacts the PlzWorker flow.

This CL is scoped only to changes for dedicated workers
(PlzDedicatedWorker) along with the minimal changes for shared
workers needed to facilitate those for dedicated workers.
Remaining changes for shared workers and test coverage for them
will be added in a followup CL (crbug.com/955476).

Test: content_browsertests
--gtest_filter=*SplitCacheDedicatedWorkerScripts*

Bug: 910713
Change-Id: Ic5f3bc496f8cb620cba4acd6f8a347de0e5423b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1713061
Commit-Queue: Cammie Smith Barnes <cammie@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Shivani Sharma <shivanisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685648}