Skip the .pydeps presubmit check on windows

BUG=600604

Review URL: https://codereview.chromium.org/1911813002

Cr-Commit-Position: refs/heads/master@{#389109}
diff --git a/PRESUBMIT_test_mocks.py b/PRESUBMIT_test_mocks.py
index 373b52c6..d9d8f95 100644
--- a/PRESUBMIT_test_mocks.py
+++ b/PRESUBMIT_test_mocks.py
@@ -20,6 +20,7 @@
     self.json = json
     self.re = re
     self.os_path = os.path
+    self.platform = sys.platform
     self.python_executable = sys.executable
     self.subprocess = subprocess
     self.files = []