Center fixpos <dialog>

LayoutTests/fast/dom/HTMLDialogElement/resources/dialog-layout.css

Index: LayoutTests/fast/dom/HTMLDialogElement/resources/dialog-layout.css
diff --git a/LayoutTests/fast/dom/HTMLDialogElement/resources/dialog-layout.css b/LayoutTests/fast/dom/HTMLDialogElement/resources/dialog-layout.css
new file mode 100644
index 0000000000000000000000000000000000000000..709b4505869c2efb85461eabfa1e523a345b70d4
--- /dev/null
+++ b/LayoutTests/fast/dom/HTMLDialogElement/resources/dialog-layout.css
@@ -0,0 +1,25 @@
+/* Remove body margin and dialog styles for easier positioning expected values */
+body {
+    height: 10000px;
+    margin: 0;
+}
+
+dialog {
+    border: 0;
+    padding: 0;
+    height: auto;
+    width: auto;
+}
+
+#absolute-div {
+    position: absolute;
+    top: 800px;
+    height: 50px;
+    width: 90%;
+}
+
+#relative-div {
+    position: relative;
+    top: 20px;
+    height: 30px;
+}