Return an infinite rect when calling RenderLayerClipper::parentClipRects on the root context layer

LayoutTests/compositing/squashing/abspos-under-abspos-overflow-scroll-expected.html

Index: LayoutTests/compositing/squashing/abspos-under-abspos-overflow-scroll-expected.html
diff --git a/LayoutTests/compositing/squashing/abspos-under-abspos-overflow-scroll-expected.html b/LayoutTests/compositing/squashing/abspos-under-abspos-overflow-scroll-expected.html
new file mode 100644
index 0000000000000000000000000000000000000000..95c21cbe3659b7aa328079e60eca6557e7173be3
--- /dev/null
+++ b/LayoutTests/compositing/squashing/abspos-under-abspos-overflow-scroll-expected.html
@@ -0,0 +1,33 @@
+<style type="text/css">
+  .first {
+    height:400px;
+    background-color: red;
+  }
+
+  .second {
+    left: 100px;
+    bottom: 0px; 
+    background-color:blue;
+    width:7px;
+    height:562px;
+    position: absolute;
+  }
+
+  .outer {
+    overflow: scroll;
+    /* FIXME: make this height: 500px to expose another bug */
+    height: 600px;
+    width: 500px;
+    position: absolute;
+    bottom:10px;
+    background-color: grey;
+  }
+</style>
+<div class="outer">
+  <div class="first"> </div>
+  <div class="second"> </div>
+</div>
+<script>
+    if (window.internals)
+        window.internals.settings.setLayerSquashingEnabled(true);
+</script>