Adam Silverstein c7b217cee9 JavaScript: when starting Backbone history, stop if previously started.
Prevent a potential error condition if Backbone history is started by a plugin or theme before core tries to start it. 

Props tfrommen.
Fixes #39612.

Built from https://develop.svn.wordpress.org/trunk@40076


git-svn-id: http://core.svn.wordpress.org/trunk@40013 1a063a9b-81f0-0310-95a4-ce76da25c4cd
2017-02-17 22:00:42 +00:00
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