fix(core): Skip native frames while searching frame URLs.

[luin]

When the exceptions are thrown from native code (see To reproduce below), InboundFilters#_getLastValidUrl() returns [native code] instead of a valid URL, causing InboundFilters#_isAllowedUrl() fail.

This PR skips native frames to solve the issue.

Package + Version

@sentry/browser@6.11.0

To reproduce

Visit https://codepen.io/luin/pen/qBmGgwv on Safari. It contains a regexp that Safari doesn't support. The error stack will show on the alert box. Notice that the first frame is [native code].

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[kamilogorek]

Thanks!