Bug 978443 - Rename NS_STYLE_CLEAR_LEFT_AND_RIGHT to NS_STYLE_CLEAR_BOTH.  r=dholbert

Rename NS_STYLE_CLEAR_LEFT_AND_RIGHT to NS_STYLE_CLEAR_BOTH.  Also fix whitespace and remove unnecessary parens in a few places and enumerate the possible break types in an assertion so that it doesn't make assumptions on the actual property values.