Customizer preview iframe should have a title

[celloexpressions]

Per ​https://make.wordpress.org/accessibility/2015/01/31/theme-switcher-accessibility-test-result/. I think "Site preview" is appropriate.

[celloexpressions]

Add a title to the customize-preview iframe

[westonruter]

I was afraid that this change might have resulted in the tooltip appearing whenever you put your mouse over any part of the preview iframe, but this doesn't seem to be the case.

[ocean90]

I don't think that the container of the iframe should get the title, rather the iframe itself. See https://core.trac.wordpress.org/ticket/28823#comment:9 for plugin details iframe.

[DrewAPicture]

See comment:2

[afercia]

New patch tries to add a title attribute for:

• main iframe (used just when the customizer overlays an admin page)
• preview iframe
• login iframe (shown in lieu of the preview iframe after session expiration)

Please check carefully, I'm not sure about a couple of things, one of them is:

iframe = $('<iframe src="' + self.previewUrl() + '" title="' + wp.customize.l10n.previewIframeTitle + '" />').hide();

why this way to select an already existing iframe? or maybe there are edge case conditions when the iframe needs to be created and suddenly hidden? Definitely don't have a deep knowledge of all the customizer code, but I'm a bit confused about that piece of code.

[ocean90]

In 31704:

Customizer: Add title attributes to iframes.

props afercia.
fixes #31202.