Lantern: Use onload handlers to create dependencies for trace events

When an image has an lengthy onload handler decode, we can connect that long task to the loading of the image. This would help with smoke testing our byte efficiency audits when using Lantern-based estimates too.

Brief investigation for onload:

* The parent event fired has `event.args.src_func: 'ImageNotifyFinished'`
* The first child event has `event.args.data.type: 'load'`, `event.name: 'EventDispatch'`

there's no URL unfortunately but we can probably connect it to an image resource based on timestamp