Fix Chromium tracing to not override app logging

Modify TraceEvent to only override tracing when it has been previously set by
Chromium, not for when other apps have set it.

BUG=651215

Review-Url: https://codereview.chromium.org/2380563003
Cr-Commit-Position: refs/heads/master@{#422016}


CrOS-Libchrome-Original-Commit: 0c54bb9b87180c4eb1fea08afae89848111ea3ad
1 file changed
tree: fce1019536d93dc6e3d108da6e288e938403afad
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/