New asserts:
- verify that after layout, the size fits the constraints
- verify that after layout, the size isn't infinite
- verify that you don't set the size in performLayout() if you have
  sizedByParent set
- verify that nobody reads your size during layout except you, or your
  parent if they said parentUsesSize:true

Fixes some bugs found by those asserts:
- RenderBlock, RenderStack, and RenderScaffold were not always setting
  parentUsesSize correctly
- RenderScaffold was setting its slot entries to null rather than
  removing them when the slot went away, which led to null derefs in
  certain circumstances

Also, rename a local variable in RenderStack.performLayout() because
it was shadowing a variable on the object itself, which was really
confusing when I first tried to debug this function...

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1213473003.