Fix IWYU inclusion of <atomic> when TRACE_EVENT is used.

The TRACE_EVENT macros use std::atomic, but TraceEvent.h already
includes <atomic> alongside the macros themselves. IWYU just can't
see through the preprocessor.

Change-Id: I6060dad2e25f3a7e30c63c2e6c4488545dcf9f96
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/793318
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
10 files changed