Prefer /-path to \-path for Goma and re-client

Bug: 1208198
Change-Id: Ieaade29caa2b3a1785ef581b98ae4027a1253ca3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2880436
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#882384}

build/toolchain/win/setup_toolchain.py

@@ -251,7 +251,7 @@ def main():
 
   def relflag(s):  # Make s relative to builddir when cwd and sdk on same drive.
     try:
-      return os.path.relpath(s)
+      return os.path.relpath(s).replace('\\', '/')
     except ValueError:
       return s