Lighthouse "Script Parse" metric is off

Parse metrics show up in Lighthouse through the "Reduce JavaScript execution time" diagnostic in the Performance audit. The code is in `bootup-time.js` (https://github.com/GoogleChrome/lighthouse/blob/b8ee28f2f4c6eb5bf02065e731e612f8f46b4e7f/lighthouse-core/audits/bootup-time.js#L134-L139) and looks for the `scriptParseCompile` task group, which Lighthouse [defines](https://github.com/GoogleChrome/lighthouse/blob/54baf91bf7832153347d8e4daf9e05da86d5d5dd/lighthouse-core/lib/task-groups.js#L55-L59) based on the following trace events:

```
scriptParseCompile: {
id: 'scriptParseCompile',
label: 'Script Parsing & Compilation',
traceEventNames: ['v8.compile', 'v8.compileModule', 'v8.parseOnBackground'],
},
```

An example report looks as follows:


"Script Parse" intends to cover "Script Parse + Compile" and should probably be renamed as such, but even then the metrics are misattributed. Lighthouse is currently underreporting "[main thread] script parse + compile" and overreporting "script evaluation" by roughly the same amount, due to “lazy” compilations (i.e. on-demand compilation of non-IIFEs) and eval compilations falling into the “evaluation” rather than “parse & compile” bucket. The exposed metric includes the `v8.parseOnBackground` trace event, although time spent on background threads explicitly does not contribute to main thread blocking time and is therefore not useful to report --- in fact, reporting it can be harmful. The current Lighthouse-reported metrics might steer developers away from code splitting & serving multiple smaller scripts (which is great for parallelization, since we then spawn multiple small tasks) as opposed to a single massive script file.

Suggested changes in Lighthouse:

1. rename "Script Parse" to "Script Parse + Compile" if that's indeed what we intend to show
2. only count main thread time spent parsing + compiling, since that's what matters

cc @addyosmani @verwaest @LeszekSwirski