Refactor script blocking cross-origin browser tests

Removes the `UpdateIncludedScriptSource` helper function in favor of
having the test HTML files load scripts based on a query param. Test
logic is updated to construct URLs with this parameter populated.

I'm honestly not sure if `UpdateIncludedScriptSource` ever worked,
because it replaced the <script> element with a URL string rather than
updating the script.src property.

Fixed: b/417489057
Change-Id: I6a6a6964f2bcfc893a5b2bd8d101b95e761fb15e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6934228
Reviewed-by: Giovanni Ortuno Urquidi <ortuno@chromium.org>
Auto-Submit: Jon Toohill <jtoohill@google.com>
Commit-Queue: Giovanni Ortuno Urquidi <ortuno@chromium.org>
Reviewed-by: Martin Pan-Verde <thesalsa@google.com>
Cr-Commit-Position: refs/heads/main@{#1514900}