The `IFrameCollection` was previously rebuilt too often. This PR tries
to address the todo, to only rebuild the IFrameCollection when
necessary.
Testing: `CSS Selector Invalidation: :has() invalidation should not be
O(n^2)` wpt-test changed status from timeout to failed.
Fixes: #38131 (IFrameCollection::validate accounts for 30% of script
time in DOM heavy scenarios)
Co-authored-by: sharpshooter_pt <ibluegalaxy_taoj@163.com>
Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>