Add render tree dump and pixel tests for statically placed position:absolute.

sky/tests/layout/position-absolute-expected.txt

Index: sky/tests/layout/position-absolute-expected.txt
diff --git a/sky/tests/layout/position-absolute-expected.txt b/sky/tests/layout/position-absolute-expected.txt
new file mode 100644
index 0000000000000000000000000000000000000000..58658574d3f166bc29b676bccb80eaeba2d39583
--- /dev/null
+++ b/sky/tests/layout/position-absolute-expected.txt
@@ -0,0 +1,21 @@
+layer at (0,0) size 800x600
+  RenderView {#document} at (0,0) size 800x600
+layer at (0,0) size 100x100
+  RenderBlock {block} at (0,0) size 100x100 [bgcolor=#FFC0CB]
+    RenderBlock {spacer} at (0,0) size 40x40 [bgcolor=#FFEFD5] [border: (5px solid #800080)]
+    RenderBlock {spacer} at (0,40) size 40x40 [bgcolor=#FFEFD5] [border: (5px solid #800080)]
+layer at (0,40) size 20x20
+  RenderBlock (positioned) {absolute} at (0,40) size 20x20 [bgcolor=#008000]
+layer at (0,150) size 100x100
+  RenderParagraph {paragraph} at (0,150) size 100x100 [bgcolor=#FFA500]
+    RenderBlock (inline-block) {spacer} at (0,0) size 40x40 [bgcolor=#FFEFD5] [border: (5px solid #800080)]
+    RenderBlock (inline-block) {spacer} at (40,0) size 40x40 [bgcolor=#FFEFD5] [border: (5px solid #800080)]
+layer at (0,194) size 20x20
+  RenderBlock (positioned) {absolute} at (0,44) size 20x20 [bgcolor=#008000]
+layer at (0,300) size 100x100
+  RenderFlexibleBox {flex} at (0,300) size 100x100 [bgcolor=#FA8072]
+    RenderBlock {spacer} at (0,0) size 40x40 [bgcolor=#FFEFD5] [border: (5px solid #800080)]
+    RenderBlock {spacer} at (40,0) size 40x40 [bgcolor=#FFEFD5] [border: (5px solid #800080)]
+layer at (40,300) size 20x20
+  RenderBlock (positioned) {absolute} at (40,0) size 20x20 [bgcolor=#008000]
+