Commits on Jan 10, 2017

1. Rework the way scroll roots are collected …

   Collect scroll roots during the collect_stacking_context phase instead
   of during display list construction. This will be useful in order to
   collect containing block scroll roots as well as to give scroll roots
   sequential ids in the future. This change also pulls stacking context
   children out of the StackingContext struct itself, which should reduce
   very slightly the memory used by the finished display list. This also
   simplifies the DisplayListBuilder because it no longer has to maintain
   a stack of ScrollRootIds and StackingContextIds and can instead just
   rely on the program stack.

   

   mrobinson committed Jan 10, 2017

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