script: Implement document's active sandboxing flag set (#39079)

Implements document's active sandboxing flags. These are currently
populated only from CSP-derived sandboxing flags for a new document,
when defined in the CSP.

Testing: 1 new pass, and some new wpt's are added to test points in the
spec where these flags influence behaviour.

Signed-off-by: Shane Handley <shanehandley@fastmail.com>

tests/wpt/meta/MANIFEST.json

@@ -403717,6 +403717,14 @@
      ]
     },
     "sandbox": {
+     "autoplay-disabled-by-csp.html.headers": [
+      "32518e57d4584de71845a9260b093c3535fc3074",
+      []
+     ],
+     "form-submission-blocked-by-sandboxing.html.headers": [
+      "1efcf8c226fac074c98d0a5a747856f532e5d84e",
+      []
+     ],
      "support": {
       "empty.html": [
        "e69de29bb2d1d6434b8b29ae775ad8c2e48c5391",
@@ -581697,6 +581705,20 @@
      ]
     },
     "sandbox": {
+     "autoplay-disabled-by-csp.html": [
+      "d7bd453a34c0e75c98c837f853c0cf492359625a",
+      [
+       null,
+       {}
+      ]
+     ],
+     "form-submission-blocked-by-sandboxing.html": [
+      "4c717a18fd8bfa9d5cb4bc5449b0f25498ccb754",
+      [
+       null,
+       {}
+      ]
+     ],
      "iframe-inside-csp.sub.html": [
       "cd402bdba0198bf763e1733004c2005614b9a542",
       [