[browser] Fix EventPipe CPU sampling stall under coarse browser timer resolution

[pavelsavara]

Fixes #129584.

Problem

On WebAssembly the EventPipe CPU sample profiler could permanently stop emitting samples. update_sample_frequency recomputes skips_per_period by dividing sample_skip_counter by ms_since_last_sample. With the coarse browser timer resolution, ms_since_last_sample can round to 0 during a burst of instrumentation callbacks. Dividing by 0 yields +Inf, which — once cast to int under WASM's saturating, non-trapping float-to-int conversion — pins skips_per_period at INT_MAX, permanently disabling sampling.

Fix

• Only recompute when wall-clock time actually advanced (ms_since_last_sample > 0); otherwise keep the previous skips_per_period.
• Clamp the averaged skip count to a sane range ([1, 10000]) so sampling stays responsive, matching the CoreCLR single-threaded sampler behavior.

This re-enables the EventPipe CPU sample Blazor tests (BlazorEventPipeTestWithCpuSamplesAOT and BlazorEventPipeTestWithCpuSamples) that were disabled by the issue.

Note

This PR description was generated with assistance from GitHub Copilot.

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Pull request overview

This PR adjusts Mono’s browser EventPipe CPU sampling skip-calculation to avoid pathological behavior when the browser’s performance.now() resolution is coarse (e.g., repeated 0 deltas), and re-enables the previously-disabled Blazor EventPipe CPU sampling tests.

Changes:

• Guard update_sample_frequency() to only recompute skips_per_period when ms_since_last_sample > 0, and clamp the computed skip range.
• Re-enable Blazor EventPipe CPU sampling tests by removing the ActiveIssue skips.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/mono/mono/eventpipe/ep-rt-mono-runtime-provider.c	Adds elapsed-time guard and clamps adaptive skip computation for single-threaded browser sampling.
src/mono/wasm/Wasm.Build.Tests/Blazor/EventPipeDiagnosticsTests.cs	Removes ActiveIssue attributes to re-enable CPU sampling test coverage.