Customizer postMessage listener throws errors if it encounters a non-JSON string

[mattwiebe]

Our postMessage listener in wp-includes/js/customize-base.js assumes that every possible postMessage message that comes in from the same domain as the Customizer must have an event.data property that is 1) a string and 2) a good candidate for JSON.parse.

Both of these assumptions are faulty, since 1) non-IE browsers can pass objects, not just strings, and 2) sending a string that can't be JSON.parse'd is a perfectly valid thing to do. As more things start to use postMessage, this will get us into more trouble. (In this case it's Google+ Follow embeds, which receive "lovely" strings like "!_{h:'I0_1385142794503'}" which JSON.parse of course chokes on.

Patch included.

[nacin]

Bad paste. Fixed in [26542].