fix msan issue by init all new members

Change-Id: If90044b99e4d8e4be3366579518faa5abc96bd31
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/263706
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Nathaniel Nifong <nifong@google.com>
3 files changed