GitLab

This CL:
- Adds wpt/upgrade-insecure-requests and worker-related
  subdirectories of wpt/mixed-content.
- Fixes the runtime flags for external/wpt/referrer-policy.
- Merges multiple README.txt into a single top-level README.md.
- Updates README.md to mention classic shared workers.

Bug: 835717, 924041
Change-Id: Ifbafe9ca89a0c45d99e23d295430df89125d71ce
Reviewed-on: https://chromium-review.googlesource.com/c/1389639Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629457}