Sorry for not giving input earlier. Are we set on having a UMA bridge? If it's generally useful, (probably yes) then let's keep going. But if it is the case that we only want the JS stats, an alternative could be to just extend IPDF_JSPLATFORM.

The IPDF_JSPLATFORM is setup as part of the formfill environment which happens after the document is loaded. I believe you can have JS run on document load, so we'd potentially miss JS firing.

We could possibly do it in the data we pass to FPDF_LIBRARY_CONFIG at library configuration time.

I think having the UMA bridge is good. I'd like to see us add some performance metrics like how long does document load take, or page rendering time.

Setting PDF bugs assigned to me back to untriaged so they can get re-assigned as needed.

I started working on this before other work came up:

PDFium CL: https://pdfium-review.googlesource.com/c/pdfium/+/27690
Chromium CL: https://chromium-review.googlesource.com/c/chromium/src/+/949665

Assigning to thestig@ for evaluating if we still want to do this.