Cleanup animator tracing

[dnfield]

• Removes the VsyncSchedulingOverhead event, since that event didn't always make sense - it records long scary bars in frames where the schedule frame did not originate from the framework, and it's not clear to me how this is useful to our users - see FrameTiming build start timestamp fix and add vsync start timestamp #20229
• Only emit Frame Request pending if we're going to call BeginFrame.
• Emit an event when we re-use the last layer tree.

Fixes flutter/flutter#113957

[flutter-dashboard]

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[dnfield]

I could probably write a test for this if we think this is something worth testing.

[zanderso]

Will this affect DevTools? @kenzieschmoll

[zanderso]

I'm not seeing the string VsyncSchedulingOverhead in the framework tree anywhere, so if DevTools isn't using it, I think it's safe to remove.

[kenzieschmoll]

LGTM from DevTools perspective. This change will not affect our tracing logic. Thank you for the tag @zanderso.

[auto-submit]

auto label is removed for flutter/engine, pr: 36982, due to - The status or check suite Linux Unopt has failed. Please fix the issues identified (or deflake) before re-applying this label.

[auto-submit]

auto label is removed for flutter/engine, pr: 36982, due to - The status or check suite Linux Unopt has failed. Please fix the issues identified (or deflake) before re-applying this label.

[zanderso]

The formatter wants the comment whitespace to be fixed.