PlzWorker: Factor out loader setup code from SharedWorkerServiceImpl to WorkerScriptFetchInitiator

As a preparation of PlzDedicatedWorker, this CL makes browser-side loader
classes for PlzSharedWorker reusable for PlzDedicatedWorker. Specifically, this
CL introduces WorkerScriptFetchInitiator that is a static class to abstract
SharedWorkerScriptFetcher and SharedWorkerScriptLoader. This doesn't change
functional behavior.

WorkerScriptFetchInitiator is now used only by SharedWorkerServiceImpl, but it
will also be used by DedicatedWorkerHost in a follow-up CL.

Next plan: Rename SharedWorkerScriptFetcher and SharedWorkerScriptLoader to
WorkerScriptFetcher and WorkerScriptLoader respectively, and remove shared
worker specific code from WorkerScriptFetchInitiator. See the design doc for
details:
https://docs.google.com/document/d/1fWsD0oIa5sNDfUFWGJZ41pDo3zzsbFGyQSNdV8nOG4I/edit?usp=sharing

Change-Id: Iae37ebb8fec6f0843df4c11fac8c9d8abe5b6667
Bug: 906991
Reviewed-on: https://chromium-review.googlesource.com/c/1345704
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610790}
5 files changed