This patch adds a change hint to signal that one of the offsets on a
frame has been changed. When processing the hint, we do one of the
following things based on the position property of the frame.
* For static frames, we ignore the offset changes completely, as they
will not change the layout.
* For relative positioned frames, this patch refactors the
nsHTMLReflowState::ComputeRelativeOffsets function so that it can be
used from other callers, and it uses that to compute the new relative
offsets, and uses the offsets computed previously to compute the new
position of the frame.
* For absolute positioned frames, we set up a fake parent reflow state
object, and then we create a new reflow state object for the frame in
question. This setup is similar to what nsFrame::BoxReflow does.
Once we have the new reflow state object, we use it to compute the
absolute offsets, and then we use the computed offsets to set the new
position of the frame. The offset computation is similar to what
nsAbsoluteContainingBlock::ReflowAbsoluteFrame does. In some cases
where it is possible for the dimensions of the frame to change based
on the offset changes, we fall back to a full reflow.
