Naively implement WorkerFetchContext::ShouldBlockFetchByMixedContentCheck()

We need more detailed check which is implemented in
MixedContentChecker::ShouldBlockFetch().
It take time to implement it for workers.
And returning false is a bit scary from the security point of view.

So I want to use MixedContentChecker::IsMixedContent() for a while.

BUG=443374

Review-Url: https://codereview.chromium.org/2895563002
Cr-Commit-Position: refs/heads/master@{#473397}
5 files changed