[PNG] Patch up for Chromium eXIf support

Hello everyone,

Previously, we marked eXIf as 'at risk' because it was only supported in
Safari and we needed support in 2+ browsers.

I created a Chromium CL to add eXIf support
<https://chromium-review.googlesource.com/c/chromium/src/+/5249880>.
Once approved, we can monitor the WPT eXIf test
<https://wpt.fyi/results/png/exif-chunk.html?label=experimental&label=master&aligned>
to make sure things are working as intended.

Once the WPT test looks good, I think we can remove the 'at risk' mark.