Initialize fBackend in Window_unix

Discovered while trying to run viewer in ASAN

Change-Id: Ic7469fdbf8cb77573c2f93503440b2137bdba783
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/423016
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
1 file changed