Remove use of SecurityContext from mixed_content_checker for workers

After this CL, FetchClientSettingsObject is used for mixed
content checker for workers instead of SecurityContext, and
WorkerFetchContext::GetSecurityContext() is removed.

This CL should cause behavior changes in mixed content check
in off-the-main-thread worker script fetch for shared and service
workers, but they are not covered by tests.
Anyway, they are still behind the flag.

Bug: 878274
Change-Id: If70038939a74da5f8a2918392b83ef11acf0fcdd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1580263
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653913}
4 files changed