Move AllowedByNosniff::MimeTypeCheck logic to FetchClientSettingsObject

This CL introduces
FetchClientSettingsObject::MimeTypeCheckForClassicWorkerScript()
and moves AllowedByNosniff::MimeTypeCheck logic to it from
WorkerClassicScriptLoader.

This CL removes most of dependencies from
WorkerClassicScriptLoader to ExeuctionContext except for
calling IsWorkerGlobalScope() and plumbing to ThreadableLoader.

This CL shouldn't change the behavior.

Bug: 880027, 794548
Change-Id: I68d31e359899faf4d5461e1be8a46db9f9bc1303
Reviewed-on: https://chromium-review.googlesource.com/c/1351929
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614461}