Fix check_package_boundaries presubmit test for Windows.

BUG=webrtc:6954
NOTRY=True

Review-Url: https://codereview.webrtc.org/2695573003
Cr-Commit-Position: refs/heads/master@{#16581}
diff --git a/tools-webrtc/check_package_boundaries_test.py b/tools-webrtc/check_package_boundaries_test.py
index 437f6e6..c59c485 100755
--- a/tools-webrtc/check_package_boundaries_test.py
+++ b/tools-webrtc/check_package_boundaries_test.py
@@ -33,6 +33,7 @@
   def log(self, build_file_path, line_number, target_name, source_file,
           subpackage):
     build_file_path = os.path.relpath(build_file_path, self.test_dir)
+    build_file_path = build_file_path.replace(os.path.sep, '/')
     self.messages.append([build_file_path, line_number, target_name,
                           source_file, subpackage])