Improve singleton example by assuming implementation is outside base/

The error messages returned by the compiler are hard to understand and
most singleton implementations live outside base/.

Also adds an example for leaky singletons because it's not clear that
DefaultSingletonTraits must still be friended.

Review-Url: https://codereview.chromium.org/2741493002
Cr-Commit-Position: refs/heads/master@{#455931}


CrOS-Libchrome-Original-Commit: 961c9d662de83d9d27547c48576e9df6ccfb5146
1 file changed
tree: d34ac957c1e2fb6bd52ce042e327307959aa81df
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/