Don't use EnsureComputedStyle in LayoutBoxModelObject.

There should be no reason to force computed style for the root element
if it doesn't have one when trying to figure out if body background is
propagated to the viewport.

Change-Id: Ibf0beb7848101bd1acc20f16a826f0787ef010b9
Reviewed-on: https://chromium-review.googlesource.com/c/1280278Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599717}

third_party/blink/renderer/core/layout/layout_box_model_object.cc

@@ -25,6 +25,7 @@
 
 #include "third_party/blink/renderer/core/layout/layout_box_model_object.h"
 
+#include "third_party/blink/renderer/core/dom/node_computed_style.h"
 #include "third_party/blink/renderer/core/frame/local_frame.h"
 #include "third_party/blink/renderer/core/frame/local_frame_view.h"
 #include "third_party/blink/renderer/core/html/html_body_element.h"
@@ -1467,8 +1468,8 @@ bool LayoutBoxModelObject::BackgroundStolenForBeingBody(
     return false;
 
   if (!root_element_style)
-    root_element_style = root_element->EnsureComputedStyle();
-  if (root_element_style->HasBackground())
+    root_element_style = root_element->GetComputedStyle();
+  if (!root_element_style || root_element_style->HasBackground())
     return false;
 
   if (GetNode() != GetDocument().FirstBodyElement())