PlzWorker: Rename SharedWorkerScript* to WorkerScript*

To make browser-side loader classes for PlzSharedWorker reusable for
PlzDedicatedWorker, this CL renames the classes as follows:

- SharedWorkerScriptFetcher => WorkerScriptFetcher
- SharedWorkerScriptLoader => WorkerScriptLoader
- SharedWorkerScriptLoaderFactory => WorkerScriptLoaderFactory
- SharedWorkerMainScriptLoadParams => WorkerMainScriptLoadParams

This doesn't change functional behavior.

See the design doc for details:
https://docs.google.com/document/d/1fWsD0oIa5sNDfUFWGJZ41pDo3zzsbFGyQSNdV8nOG4I/edit?usp=sharing

Bug: 906991
Change-Id: If25f463655f2f53a533a6c1504f57f05937860a1
Reviewed-on: https://chromium-review.googlesource.com/c/1350018
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611084}
24 files changed