LibWeb: Fire iframe load event on document close

[tcl3]

This matches the behavior of other browsers.

This gets rid of many timeouts for the HTML parser WPT tests in the html/syntax/parsing subdirectory

These are the results of running ./Meta/WPT.sh run html/syntax/parsing:

Before:

  Ran 208 tests finished in 213.4 seconds.
  • 59 ran as expected. 0 tests skipped.
  • 22 tests crashed unexpectedly
  • 103 tests timed out unexpectedly
  • 24 tests had unexpected subtest results

After:

Ran 208 tests finished in 173.5 seconds.
  • 87 ran as expected. 0 tests skipped.
  • 38 tests crashed unexpectedly
  • 2 tests timed out unexpectedly
  • 81 tests had unexpected subtest results