Fix audit_non_blink_usage.py on Windows.

Pathnames are delimited with \ on Windows by default, while the script
only expects /

Change-Id: Ie15090dd45440dbb4eaad1e5feb48d18a0ed971f
Reviewed-on: https://chromium-review.googlesource.com/1225933Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591217}

third_party/blink/tools/audit_non_blink_usage.py

@@ -521,6 +521,8 @@ def check(path, contents):
         A list of line number, disallowed identifier tuples.
     """
     results = []
+    # Because Windows.
+    path = path.replace('\\', '/')
     basename, ext = os.path.splitext(path)
     # Only check code. Ignore tests.
     # TODO(tkent): Remove 'Test' after the great mv.