This change creates a new struct `IFrameCollection` that is used to
cache the list of `<iframe>`s in a `Document` as long as the
`Document`'s DOM has not changed. This prevent constantly iterating the
entire DOM during *update the rendering*, which runs up to 60 times per
second as well as for other operations.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
