Revert CSP application to `DOMParser.parseFromString`.

This patch partially reverts [1], which began applying the context document's
CSP to the document fragment produced from `DOMParser.parseFromString`. This
application broke the expectations of GMail's dynamic email feature, and we
ought to pull it out until we understand whether our CSP implementation is
wrong, or whether GMail is wrong.

[1]: https://chromium-review.googlesource.com/c/chromium/src/+/1917532

Bug: 1030830, 951536
Change-Id: Id7e86ca85df8790bb58fda15827a3f34e00239fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1975710Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726326}

third_party/blink/renderer/core/xml/dom_parser.cc

@@ -46,8 +46,7 @@ Document* DOMParser::parseFromStringInternal(const String& str,
       type,
       DocumentInit::Create()
           .WithContextDocument(context_document_)
-          .WithOwnerDocument(context_document_)
-          .WithContentSecurityPolicyFromContextDoc(),
+          .WithOwnerDocument(context_document_),
       false);
   doc->SetContent(str);
   doc->SetMimeType(AtomicString(type));

third_party/blink/web_tests/external/wpt/trusted-types/trusted-types-createHTMLDocument.tentative-expected.txt

+This is a testharness.js-based test.
+PASS Trusted Type assignment is blocked. (document)
+PASS Trusted Type instances created in the main doc can be used. (document)
+PASS Trusted Type assignment is blocked. (createHTMLDocument)
+PASS Trusted Type instances created in the main doc can be used. (createHTMLDocument)
+FAIL Trusted Type assignment is blocked. (DOMParser) assert_throws: function "_ => { doc.createElement("script").textContent = "2+2"; }" did not throw
+FAIL Trusted Type instances created in the main doc can be used. (DOMParser) assert_throws: function "_ => { doc.body.innerHTML = "world"; }" did not throw
+PASS Trusted Type assignment is blocked. (XHR)
+PASS Trusted Type instances created in the main doc can be used. (XHR)
+PASS Install default policy.
+PASS Default policy applies. (document)
+PASS Default policy applies. (createHTMLDocument)
+PASS Default policy applies. (DOMParser)
+PASS Default policy applies. (XHR)
+Harness: the test ran to completion.
+