fChildren/fChildCount was replaced with fChildVector and Chromium
migrated to the new API a while back, it's now safe to remove the
older interface.
Bug: chromium:607777
Change-Id: I7311d3b51f1b71209dcc024ae51637536d560619
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/297260
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>