Fix background show logic for UI compositor

The logic that determined whether or not to show the background
of the root layer is no longer applicable due to the toolbar layer
always being a parent to the content. This change updates the
background visibility logic in for StaticTabSceneLayer to look at
the root layer.

BUG=615365

Review-Url: https://codereview.chromium.org/2040513002
Cr-Commit-Position: refs/heads/master@{#399311}
(cherry picked from commit ab5f51e)

Review URL: https://codereview.chromium.org/2060653002 .

Cr-Commit-Position: refs/branch-heads/2743@{crosswalk-project#325}
Cr-Branched-From: 2b3ae3b-refs/heads/master@{#394939}

chrome/browser/android/compositor/scene_layer/static_tab_scene_layer.cc

@@ -27,8 +27,8 @@ StaticTabSceneLayer::~StaticTabSceneLayer() {
 }
 
 bool StaticTabSceneLayer::ShouldShowBackground() {
-  scoped_refptr<cc::Layer> parent = layer_->parent();
-  return parent.get() && parent->bounds() != layer_->bounds();
+  scoped_refptr<cc::Layer> root = layer_->RootLayer();
+  return root && root->bounds() != layer_->bounds();
 }
 
 SkColor StaticTabSceneLayer::GetBackgroundColor() {