Add tests and settings plumbing for accelerated fixed root background

LayoutTests/compositing/fixed-body-background-positioned-expected.txt

Index: LayoutTests/compositing/fixed-body-background-positioned-expected.txt
diff --git a/LayoutTests/compositing/fixed-body-background-positioned-expected.txt b/LayoutTests/compositing/fixed-body-background-positioned-expected.txt
new file mode 100644
index 0000000000000000000000000000000000000000..8b0cfe4d218fdbb67ed9c8e17c9e005286b3bf43
--- /dev/null
+++ b/LayoutTests/compositing/fixed-body-background-positioned-expected.txt
@@ -0,0 +1,30 @@
+(GraphicsLayer
+  (children 2
+    (GraphicsLayer
+      (bounds 785.00 600.00)
+      (children 1
+        (GraphicsLayer
+          (position 0.00 -200.00)
+          (children 1
+            (GraphicsLayer
+              (bounds 785.00 3700.00)
+              (children 1
+                (GraphicsLayer
+                  (bounds 785.00 3700.00)
+                  (contentsOpaque 1)
+                  (drawsContent 1)
+                )
+              )
+            )
+          )
+        )
+      )
+    )
+    (GraphicsLayer
+      (position 785.00 0.00)
+      (bounds 15.00 600.00)
+      (contentsOpaque 1)
+    )
+  )
+)
+