Only allocate a foreground layer for a CLM if there are composited negative-z-index-children.

Otherwise, those children can be rendered as pat of the software rendering of the current
layer and its non-compositing descendants.

BUG=

Review URL: https://codereview.chromium.org/535553002

git-svn-id: svn://svn.chromium.org/blink/trunk@181424 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/LayoutTests/compositing/geometry/bounds-ignores-hidden-dynamic-negzindex-expected.txt

@@ -11,13 +11,7 @@
         {
           "transformOrigin": [50, 50],
           "bounds": [500, 250],
-          "drawsContent": true,
-          "children": [
-            {
-              "bounds": [500, 250],
-              "drawsContent": true
-            }
-          ]
+          "drawsContent": true
         },
         {
           "children": [
@@ -25,13 +19,7 @@
               "position": [0, 250],
               "transformOrigin": [50, 50],
               "bounds": [150, 150],
-              "drawsContent": true,
-              "children": [
-                {
-                  "bounds": [150, 150],
-                  "drawsContent": true
-                }
-              ]
+              "drawsContent": true
             },
             {
               "bounds": [792, 100],

third_party/WebKit/LayoutTests/compositing/geometry/bounds-ignores-hidden-expected.txt

@@ -12,13 +12,7 @@
           "transformOrigin": [-10, -10],
           "bounds": [50, 50],
           "contentsOpaque": true,
-          "drawsContent": true,
-          "children": [
-            {
-              "bounds": [50, 50],
-              "drawsContent": true
-            }
-          ]
+          "drawsContent": true
         }
       ]
     }

third_party/WebKit/LayoutTests/css3/compositing/mix-blend-mode-2nd-stacking-context-composited-expected.txt

@@ -15,13 +15,7 @@
               "bounds": [160, 90],
               "contentsOpaque": true,
               "drawsContent": true,
-              "backgroundColor": "#FF0000",
-              "children": [
-                {
-                  "bounds": [160, 90],
-                  "drawsContent": true
-                }
-              ]
+              "backgroundColor": "#FF0000"
             },
             {
               "bounds": [800, 600],

third_party/WebKit/Source/core/rendering/compositing/RenderLayerCompositor.cpp

@@ -753,11 +753,13 @@ bool RenderLayerCompositor::clipsCompositingDescendants(const RenderLayer* layer
     return layer->hasCompositingDescendant() && layer->renderer()->hasClipOrOverflowClip();
 }
 
-// If an element has negative z-index children, those children render in front of the
+// If an element has composited negative z-index children, those children render in front of the
 // layer background, so we need an extra 'contents' layer for the foreground of the layer
 // object.
 bool RenderLayerCompositor::needsContentsCompositingLayer(const RenderLayer* layer) const
 {
+    if (!layer->hasCompositingDescendant())
+        return false;
     return layer->stackingNode()->hasNegativeZOrderList();
 }