Can't apply element hiding rule to the element inside IFRAME

@Alex-302 commented on [Sat Sep 09 2017](https://github.com/AdguardTeam/AdguardBrowserExtension/issues/852)

Original issue
https://github.com/AdguardTeam/AdguardFilters/issues/6595#issuecomment-328218629


Does not work: `t3n-de.cdn.ampproject.org##amp-iframe`
works: `google.de##amp-iframe`

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@ameshkov commented on [Sun Sep 17 2017](https://github.com/AdguardTeam/AdguardBrowserExtension/issues/852#issuecomment-330051035)

The thing is that this is how it is supposed to work in Safari.

`if-domain`/`unless-domain` are checked against the window domain, frame domain is ignored.

We can fix this if we use `url-filter` instead of the `if-domain`, but it's not good from the performance point of view.