Placeholders implementation + unit tests

Change-Id: I4e87a6927cbd6ef087b72aaa4b2155c1859589b3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/235677
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Julia Lavrova <jlavrova@google.com>