WebSocket: Implement MixedContentChecker::IsWebSocketAllowed()

This is a preparation CL for enabling to establish WebSocket connection off the
main thread.

In the current implementation, mixed content checks for WebSocket are bound
with LocalFrame. This blocks off-main-thread WebSocket.

To unblock it, this CL introduces IsWebSocketAllowed() in MixedContentChecker
that can do the checks based on both LocalFrame and WorkerGlobalScope.

Bug: 825740
Change-Id: I2cb951b4c23b072605602d6a5dc4a38e28ac5bd5
Reviewed-on: https://chromium-review.googlesource.com/983076
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548392}
10 files changed