core(runner): store gather timing on artifacts

[adamraine]

Proposal to fix the timing issues in #13569.

• Taking a position on core(runner): independent gather and audit functions #13569 (comment) , lh:runner:gather will always reflect the time spent gathering the page, and never the time to load artifacts from disk.
• There will always be a lh:runner:gather in artifacts.Timing
• Sample artifacts will have unique startTime for each timing entry so they aren't deduped in Runner._getTiming:

lighthouse/lighthouse-core/runner.js

Lines 200 to 202 in 62848fb

	// As entries can share a name, dedupe based on the startTime timestamp
	].map(entry => /** @type {[number, PerformanceEntry]} */ ([entry.startTime, entry]));
	const timingEntries = Array.from(new Map(timingEntriesKeyValues).values())

[paulirish]

yup its great