[web] Leave blob URLs untouched in TT policy.

[ditman]

Very simple tweak to flutter.js that allows blob URLs to be used as entrypointUrls when loading flutter (this is needed by dartpad)

Testing

I had to test this locally by spinning up a flutter app with something like:

<script>
  let initSrc = `let actualMain = document.createElement("script");
document.head.appendChild(actualMain);
actualMain.id = "injected_from_a_blob";
actualMain.src = "main.dart.js";`;
  let blobInit = new Blob([...initSrc]);
  let blobUrl = URL.createObjectURL(blobInit);
</script>
<script>
  window.addEventListener('load', function(ev) {
    // Download main.dart.js
    _flutter.loader.loadEntrypoint({
      entrypointUrl: blobUrl,
      onEntrypointLoaded: function(engineInitializer) {
        engineInitializer.autoStart();
      }
    });
  });
</script>

Issues

• Fixes Support Blobs in TrustedTypesPolicy flutter#141329

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[ditman]

/cc @johnpryan

[yjbanov]