Pascal Birchler 56ebd53549 Embeds: Improve how iframes are loaded after being initially hidden.
Use a more accessible way to initially hide the iframe. After that, only display an iframe when it was successfully loaded.

Merge of [36648] and [36708] to the 4.4 branch.

Fixes #35894.

git-svn-id: https://develop.svn.wordpress.org/branches/4.4@37093 602fd350-edb4-49c9-b593-d223f7449a82
2016-03-30 10:56:44 +00:00
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