[tracing] Don't pop context from an empty stack.

Small fix to avoid popping an empty stack. Tracing on Android crashes
on that almost every time.

Review-Url: https://codereview.chromium.org/1927963003
Cr-Commit-Position: refs/heads/master@{#390744}


CrOS-Libchrome-Original-Commit: 9ea9470a13ec20a9480ae268af609599235ef4a2
1 file changed
tree: 826f37e783396f9fea29e916511efad5cadf977f
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/