Fix PRESUBMIT for long paths

Check all testable files, not just source files.

Bug: 1018801
Change-Id: I94e1fe78e57304dacaa59bb12d4004cffc7d6580
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1893503Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711505}

PRESUBMIT.py

@@ -4317,7 +4317,7 @@ def _CheckForLongPathnames(input_api, output_api):
   This causes issues on Windows.
   """
   problems = []
-  for f in input_api.AffectedSourceFiles(None):
+  for f in input_api.AffectedTestableFiles():
     local_path = f.LocalPath()
     # Windows has a path limit of 260 characters. Limit path length to 200 so
     # that we have some extra for the prefix on dev machines and the bots.