fix(metrics): Check for cls entry sources (#3775)
fix(metrics): Check for cls entry sources (#3775)

The LayoutShift.sources field may not exist in certain browsers, even if
the LayoutShift API is there. This PR patches logic that adds CLS data
to make sure that `._clsEntry.sources` exists.

Ref: https://developer.mozilla.org/en-US/docs/Web/API/LayoutShift
getsentry/raven-js,getsentry/raven-js,getsentry/raven-js,getsentry/raven-js