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[ci skip]

 * forward withCerdentials in spf.nav.request.send
 * thread through nav.load

Allow setting withCredentials in spf xhr options

On platforms with significant clock skew between `performance.now()`
and `(new Date()).getTime()`, tests that do not use `spf.now()` are
unreliable.  Fix this by using `spf.now()` in tests.

Also, standardize on `(new Date()).getTime()` instead of
`(+new Date())`, which was only being used in tests.

Improve test reliability on platforms with clock skew

Tighter property checking in JsCompiler is warning about the `username`
and `password` property access on `a` elements.  Temporarily suppress
these warnings until the default externs can be fixed or the property
accesses are removed.

Suppress "non-existent property" warnings in spf.url

This will allow for dynamic header values to be set in the config by
simply having an object with a `toString` method.

Force header values in the request to be a string.

This is safe to ignore in SPF code.

Remove deprecated flags

Suppress strictMissingDeps in SPFJS.