Worker: Clean up WebSharedWorkerImpl

For off-the-main-thread shared worker top-level script loading, this CL cleans
up startup sequence in WebSharedWorkerImpl.

Specifically, this CL...

- factors out WorkerClients creation and WorkerThread creation from
  ContinueOnScriptLoaderFinished() into CreateWorkerClients() and
  StartWorkerThread(),
- uses FetchClientSettingsObjectSnapshot as much as possible instead of directly
  asscessing Document in order to remove WorkerShadowPage later.

Change-Id: I9a9c864f6234c6bd5042fabcdbc3fd4f1cbacd7c
Bug: 835717
Reviewed-on: https://chromium-review.googlesource.com/c/1358110
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613422}
2 files changed