Round to microseconds when comparing slice ends
Round to microseconds when comparing slice ends

This CL copies the telemetry change https://codereview.chromium.org/144743002.

TEST=tracing.trace_model.slice_group.subSlicesBuilderTolerateFPInaccuracy
R=nduca@chromium.org

Review URL: https://codereview.appspot.com/56080043
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