Bug 1077849 - Use eRestyle_ForceDescendants for all cases of rebuilding the rule tree.  r=heycam

Bug 931668 added the eRestyle_ForceDescendants case for all cases that
go through RebuildAllStyleData, but there is another case that calls
DoRebuildAllStyleData directly, and we should cover that too.

This is needed because when we're rebuilding the rule tree, we need to
create new style contexts for all frames.

I don't know of any bugs caused by this, but it seems worth fixing.