Update: Jan 2017:
Stop using child-src and begin using frame-src again.
In an effort to create even more confusion, CSP Level 3 has undeprecated frame-src and actually re-appointed it as the preferred way to achieve this. While child-src is still supported frame-src is once again preferred.
Old post
frame-src is deprecated, but it was only recently made so in CSP Level 2 and not all browsers are up to the latest version of the spec.
The best approach at the moment for maximum browser compatibility is to include both child-src and frame-src with identical values. Browsers that only support the original CSP specification will use frame-src while newer ones will use child-src.
That developer console warning is of no consequence and merely informational. I would suggest you ignore it for now, because a year from now you may very well see one saying that frame-src is deprecated.
At this time, I ensure both are used when this is needed and intend to stop providing frame-src in January of 2017.
CSP Level 2 support:
- Firefox – Pending
- Chrome – 40+
- Opera – 27+
- Microsoft Edge – Pending
- Safari – 10+