Fix painting of position:absolute inside display: paragraphs.

We only need to walk the immediate children of the paragraph.
The other inline children of the paragraph will get paint called
on them and they will then appropriately add the self painting
layers to the array inside their paint calls.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/945223002

engine/src/flutter/tests/layout/position-absolute-pixels-expected.sky

@@ -1,7 +1,7 @@
 <sky>
   <import src="../resources/run-after-display.sky"/>
 
-  <img src="http://storage.googleapis.com/mojo/sky-pngs/779342ae094f03cdac369ade75c087d56c742669" />
+  <img src="http://storage.googleapis.com/mojo/sky-pngs/df9f2bee2247645ae83ed672858ee47fb239f70a" />
 
   <script>
   import "dart:sky";