Only call OS::GetCurrentThreadCPUMicros when a TimelineDurationScope is enabled
Only call OS::GetCurrentThreadCPUMicros when a TimelineDurationScope is enabled

OS::GetCurrentThreadCPUMicros is expensive on Mac.

Fixes #26423

R=iposva@google.com

Review URL: https://codereview.chromium.org/1975713003 .

dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk