Fix command line option checking in preprocess_idls.pl
Fix command line option checking in preprocess_idls.pl

Fix optparse usage: check if required option is passed by checking
options.option_name is None, rather than hasattr(), as always hasattr.

(Switch back to optparse from argparse was last-minute, and mandatory
 options are discouraged in optparse, hence mistake.)

Follow-up to:
Issue 15423003: Support --write-file-only-if-changed option in run-bindings-tests
https://codereview.chromium.org/15423003/

BUG=241016

Review URL: https://chromiumcodereview.appspot.com/15300020

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@150731 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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