* Get the computed -moz-binding directly from the style context which is faster than getComputedStyle
* Speed up computed style by not QI'ing so much
* Cache the computed style object so we don't keep re-alloc'ing memory for it.
* Shrink ROCSSPrimitiveValue.
* Don't get computed style of non-existant properties.
* Use nsSize instead of nsRect when we only care about height and width
* Other random code cleanup of computed style
Bug 155525
r=bzbarsky
sr=jst
