Breakdown of main thread costs

[paulirish]

We're interested in drilling into the main thread costs from the trace a few directions.

There's a few cost breakdowns that we want to try:

• By File
• By Subdomain
• By Event Name
• By Event Category
• later: By Frame (top & iframes)

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There's similarity to the bottom-up view in Timeline and Tracing's filesize groupings and WPT's processing breakdown:

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For comprehensiveness: there are a few groupings out there of tasks into categories:

• devtools: devtools-frontend/TimelineModel.js and devtools-frontend/TimelineUIUtils.js
• tracing: catapult/chrome_user_friendly_category_driver.html
• webpagetest: webpagetest/breakdownTimeline.php (older and somewhat incomplete)
• devtools v2: Issue 1451543002: DevTools: expand timeline categories - Code Review

[paulirish]

Unanswered questions:

• What category grouping do we use?
• Frame grouping, how? Lots of questions here, and best to sync with caseq/skyostil on frame blaming, etc.
• Can we match the same file attribution done in devtools? (Forced layouts attributed to script that forced it, etc)

[paulirish]

related: #3105 #1695 #2535

[paulirish]

This is completed now.