Make CheckedArithmetic slightly more sane
https://bugs.webkit.org/show_bug.cgi?id=112178

Reviewed by Geoffrey Garen.

Add an enum type for safeGet rather than using a bool,
and correctly handle unsigned * unsigned, when one value
is 0.

* wtf/CheckedArithmetic.h:
(WTF::ENUM_CLASS):
(WTF):
(WTF::operator+):
(WTF::operator-):
(WTF::operator*):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@145580 268f45cc-cd09-0410-ab3c-d52691b4dbfc