Basic changes to get XPS drawing text.

Basic maintenance to get basic things working. Most of the motivation is
to use better ownership and iteration patterns.

Change-Id: If8ffe0c7b2bddd55259ac8fa119fc9e233b9249d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/236616
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Hal Canary <halcanary@google.com>