Make autotest.py support forward slash on Windows

Currently, autotest.py doesn't support the forward slash '/' as a path
separator on Windows. For example:

Succeeds:
python3 tools/autotest.py -C out/bug base\at_exit_unittest
--gtest_filter=*Exit*

Fails:
python3 tools/autotest.py -C out/bug base/at_exit_unittest
--gtest_filter=*Exit*

This is because RecursiveMatchFilename() considers a file a match with
the requested filename if the requested filename is a substring of the
file's full path. On Windows, file paths as returned by os.scandir()
use '\\' as a path separator, so a path using '/' will never match
(e.g. 'base/at_exit_unittest' is not a substring of
'C:\\src\\chromium\\src\\base\\at_exit_unittest.cc').

With this change, either type of path separator works on Windows.

Change-Id: Ic21eff291d86e7ca994faeda807f6aeaff737562
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2181744
Commit-Queue: Jesse McKenna <jessemckenna@google.com>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#766500}

tools/autotest.py

@@ -125,6 +125,12 @@ def RecursiveMatchFilename(folder, filename):
 
 
 def FindMatchingTestFile(target):
+  if sys.platform.startswith('win32') and os.path.altsep in target:
+    # Use backslash as the path separator on Windows to match os.scandir().
+    if DEBUG:
+      print('Replacing ' + os.path.altsep + ' with ' + os.path.sep + ' in: '
+            + target)
+    target = target.replace(os.path.altsep, os.path.sep)
   if DEBUG:
     print('Finding files with full path containing: ' + target)
   results = RecursiveMatchFilename(SRC_DIR, target)