Teach make_runtime_features.py how to clobber directories in the way of its outputs.
Teach make_runtime_features.py how to clobber directories in the way of its outputs.

This works around a previous gyp error leaving turd directories on some bots
blocking the generation of RuntimeEnabledFeatures.* and requiring a clobber.
This change makes the manual clobber unecessary.

TBR=dpranke

Review URL: https://codereview.chromium.org/14555004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@149387 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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