[Mac] Font fallback when drawing download badge in dock.
[Mac] Font fallback when drawing download badge in dock.

Passing nil to -setObject:forKey: causes an NSException.  Even though
Helvetica should be a system font, so Helvetica-Bold should be super
safe, this has been happening in the wild.  Fall back to a more
generic font, then if that fails fall back to not drawing the badge
right.

[Maybe someone will report the glitch and file a bug so we can see
what's wrong.]

BUG=89086
TEST=none


Review URL: http://codereview.chromium.org/7346023

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@92413 0039d316-1c4b-4281-b951-d872f2087c98

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