Check unified strict CSP for getAllScreensMedia

The CSP of a document is derived from mutiple CSP definitions (e.g.
through HTTP headers or meta tags). All these definitions are
merged by taking the strictest requirement for each directive.
In a previous CL, we added a strict CSP check for getAllScreensMedia
assuming that strict CSP will be provided in a single
definition. This was too restrictive and this restriction is
loosened up by checking the unified CSP.

Bug: b/332485961
Change-Id: Ic7ffa024b08ce1b05516d1c4b6e88f2ab14f033f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5412654
Reviewed-by: Ted (Chromium) Meyer <tmathmeyer@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Simon Hangl <simonha@google.com>
Cr-Commit-Position: refs/heads/main@{#1282411}