[tracing] Add task execution event for sequenced worker pool
[tracing] Add task execution event for sequenced worker pool

The sequenced worker pool tasks use a flow event while other task
runners add without flow. So, instead of using TRACE_TASK_EXECUTION
macro, add just the SCOPED_TASK_EXECUTION_EVENT part for these tasks.

BUG=600440

Review-Url: https://codereview.chromium.org/1929913002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#393976}

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