* benchmark updating many child layers

* Add path construction benchmark

* Fix analyzer warnings

* Split file into test+generated code. Add comment

* Add license to new file

* remove unused import

* extract engine sub-metrics; change reported metrics

- Extract sub-metrics reported by the Web engine: "preroll_frame", "apply_frame".
- Add a concept of unreported metrics: displayed on the benchmark UI, but not on the dashboard.
- Make "sceneBuildDuration" and "windowRenderDuration" unreported, which are too fine-grained. They are included in "drawFrameDuration" already.
- Report outlier ratio instead of outlier average. The ratio is more useful of the two.

* web benchmarks: separate outliers to reduce noise; add visualization

Collect chrome://tracing data in Web benchmarks

* [Web] Add Material Card Infinite Scroll benchmark

Adds a benchmark that makes an infinite list of Material cards
and scrolls it. This benchmark exercises more heavyweight rendering
like shadows and clipping and paths.

* Fix analyzer warnings. Respond to comments

Initial web benchmark harness and 3 benchmarks