Don't mark a positioned object for layout when we've just laid it out.

LayoutTests/fast/block/positioning/setting-layout-on-posobjs-while-laying-them-out.html

Index: LayoutTests/fast/block/positioning/setting-layout-on-posobjs-while-laying-them-out.html
diff --git a/LayoutTests/fast/block/positioning/setting-layout-on-posobjs-while-laying-them-out.html b/LayoutTests/fast/block/positioning/setting-layout-on-posobjs-while-laying-them-out.html
new file mode 100644
index 0000000000000000000000000000000000000000..5950a09bcb9fc5b2b284c09012ec61f062ae63ad
--- /dev/null
+++ b/LayoutTests/fast/block/positioning/setting-layout-on-posobjs-while-laying-them-out.html
@@ -0,0 +1,16 @@
+<style>

[leviw_travelin_and_unemployed, 2015/05/19 18:32:12]

Nit: doctype :p

+div {
+    position: fixed
+    }

[leviw_travelin_and_unemployed, 2015/05/19 18:32:12]

Nit: Un-indent this brace

+</style>
+crbug.com/477940: Don't set positioned objects for layout while laying out positioned objects. This test should not ASSERT.
+<div>
+    <div>
+<script>
+    if (window.testRunner)
+        testRunner.dumpAsText();
+    thead = document.createElement("thead");
+    document.body.appendChild(thead);
+    document.body.offsetTop;
+    document.body.removeChild(thead);
+</script>