Fix mojob.py on Windows.
Fix mojob.py on Windows.

Move the logic for deciding whether to use clang, to somewhere where
Config has already detected the target OS.

(As a drive-by, change a bunch of 's to "s in mojob.py, for
consistency.)

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/760683002

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