Fix path separators on Windows in zip_utils

Bug: skia:7077
Change-Id: I5119a7d6c7cd2317129d40ae3e0997dfc9978c25
Reviewed-on: https://skia-review.googlesource.com/52664
Reviewed-by: Ravi Mistry <rmistry@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
1 file changed