Fix SkSL dehydration on Windows machines

Replace SkOSPath with bespoke code that handles both kinds of slashes.

Change-Id: I291be5456631e6abe88492f7aea9f543f37c59f1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/569622
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed