Properly detect scheme-less URL's in wp_enqueue_script

[evansolomon]

We should detect script URL's with leading // and load either HTTP or HTTPS depending on the requesting page. This would more accurately reflect the behavior of the src parameter of the default script tag in HTML.

Several third party scripts use this format to load securely only when required.

There's no reason for a relative path to start with // so this should not break any pre-existing use cases.

[koopersmith]

It sounds like there's no downside to this. wp_enqueue_script should reflect browser behavior, especially since we encourage its use as a best practice.

[SergeyBiryukov]

#16560