Consider the following scenario:
- Tracing is started.
- API call is made (e.g. page.waitForResponse), almost finishes, and
enters onAfterCall where it starts a snapshot.
- tracing.stopChunk is called, and waits for existing actions to finish.
However, it does so by calling onAfterCall one more time.
- tracing.stopChunk removes instrumentation listener and returns
to the client.
- Client starts zipping files.
- Original API call finishes the snapshot and saves it to the trace file.
This results in trace file being written to while the zip is still working.
We never marked empty stylesheets as "stale", so we never computed
css text for them. This prevented node reuse, because empty string
is not equal to undefined.
