Fix invalid path concatenations in Canvas WPT generator
Paths were concatenated with hard-coded forward slashes, which is not
valid on Windows.
Bug: 364549423
Change-Id: I0129e360ee3066c8957c49090f1546cce27e9903
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5839987
Reviewed-by: Andres Ricardo Perez andresrperez@chromium.org
Commit-Queue: Jean-Philippe Gravel jpgravel@chromium.org
Cr-Commit-Position: refs/heads/main@{#1351567}