CSP3: Implement 'worker-src'.

As a drive-by, this also renames `allowChildFrameFromSource` to
`allowFrameFromSource`, and `allowChildContextFromSource` to
`allowWorkerFromSource` to reflect their usage.

Intent to Ship: https://groups.google.com/a/chromium.org/d/msg/blink-dev/1UkZE-vOROc/gEj7psewAAAJ

BUG=662930

Review-Url: https://codereview.chromium.org/2480303002
Cr-Commit-Position: refs/heads/master@{#431228}
26 files changed