Bug 1072724 - Support showing more information about style structs in restyle logs. r=dbaron

The MOZ_DEBUG_RESTYLE_STRUCTS environment variable can be set to a comma-
separated list of style struct names.  When restyle logging is enabled,
this will cause the style context tree -- showing cached style struct
pointers for those structs specified -- to be logged before each
individual restyle is processed.  It will also show the struct pointer
values involved when swapping structs between style contexts.

For example, set MOZ_DEBUG_RESTYLE_STRUCTS=Font,UserInterface to show
the cached nsStyleFont and nsStyleUserInterface pointers on the style
contexts involved in the restyle process.