Record CPU traces from screenshot runs #791
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This builds on #790 and #789, happy to rebase as needed.
When debugging performance problems with screenshots, it's helpful to have a flamegraph of what's taking time.
It's difficult to mount a debugger into the internal story isolate. This PR enables a
--tracecli option that records a Chromium CPU trace of the screenshot and saves it alongside the screenshotfilename.pngasfilename_trace.json.This trace can be imported into Chrome's Performance Flamegraph or https://ui.perfetto.dev/ for viewing.
This is especially helpful for situations like:
Despite the screenshot not succeeding, the trace of the last attempt will still be written, and we can plainly see that this particular screenshot continues work forever, never giving the metrics a chance to stabilise: