Added needed call to close() in _HexDigest.

BUG=561763
TBR=dpranke@chromium.org

Review URL: https://codereview.chromium.org/1873493003

Cr-Original-Commit-Position: refs/heads/master@{#385910}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 91e5254fa5611aedf5cb9c540a85aef0a19b92e3

win/copy_cdb_to_output.py

@@ -23,6 +23,7 @@ def _HexDigest(file_name):
   while len(buf) > 0:
     hasher.update(buf)
     buf = afile.read(blocksize)
+  afile.close()
   return hasher.hexdigest()