Issue 493505764: LayoutShift paint timing sampler

Chromium #493505764 WICG #128 Red/green checks for paint-aligned timing and task-end deferral

Goal

Paint path: one ordinary layout shift should report paintTime > 0 and startTime == paintTime.

Forced-layout probe: a shift created in the middle of a long task should not keep the mid-task timestamp.

In the forced-layout probe, green means startTime is deferred to paint or task end and does not stay at the forced-layout timestamp.

Include forced layout work before paint-path shift

Forced-layout probe busy loop (ms):

Optional no-paint probe (best-effort): revert top to 0 before task end

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