Make the DDLTileHelper skip uninitialized tiles

This simplifies debugging DDL rendering by allowing arbitrary tiles
to be dropped.

Change-Id: If1341691ef29159a5d48cf1ae40b6f585a7e21a7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/352576
Reviewed-by: Adlai Holler <adlai@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed