mix-blend-mode implementation for accelerated layers

cc/trees/layer_tree_host_common.cc

Index: cc/trees/layer_tree_host_common.cc
diff --git a/cc/trees/layer_tree_host_common.cc b/cc/trees/layer_tree_host_common.cc
index 784b3db94a85cb900fe66c22cb4b9fd0aa4411b7..453a6f348abca713a2f92e97dad4cf0ab804ad78 100644
--- a/cc/trees/layer_tree_host_common.cc
+++ b/cc/trees/layer_tree_host_common.cc
@@ -587,6 +587,19 @@ static bool SubtreeShouldRenderToSeparateSurface(
     return true;
   }
 
+  // If the layer has blending.
+  // TODO(rosca): this is temporary, until blending is implemented for other
+  // types of quads than RenderPassDrawQuad. Layers having descendants that draw
+  // content will still create a separate rendering surface.
+  if (!layer->uses_default_blend_mode()) {
+    TRACE_EVENT_INSTANT0(
+        "cc",
+        "LayerTreeHostCommon::SubtreeShouldRenderToSeparateSurface blending",
+        TRACE_EVENT_SCOPE_THREAD);
+    DCHECK(!is_root);
+    return true;
+  }
+
   // If the layer clips its descendants but it is not axis-aligned with respect
   // to its parent.
   bool layer_clips_external_content =
@@ -629,6 +642,19 @@ static bool SubtreeShouldRenderToSeparateSurface(
   // be used as a contributing surface in order to apply correctly.
   //
 
+  // If the layer has isolation.
+  // TODO(rosca): to be optimized - create separate rendering surface only when
+  // the blending descendants might have access to the content behind this layer
+  // (layer has transparent background or descendants overflow).
+  // https://code.google.com/p/chromium/issues/detail?id=301738
+  if (layer->is_root_for_isolated_group()) {
+    TRACE_EVENT_INSTANT0(
+        "cc",
+        "LayerTreeHostCommon::SubtreeShouldRenderToSeparateSurface isolation",
+        TRACE_EVENT_SCOPE_THREAD);
+    return true;
+  }
+
   // If we force it.
   if (layer->force_render_surface())
     return true;
@@ -2034,6 +2060,16 @@ static void CalculateDrawPropertiesInternal(
       return;
     }
 
+    // Layers having a non-default blend mode will blend with the content
+    // inside its parent's render target. This render target should be
+    // either root_for_isolated_group, or the root of the layer tree.
+    // Otherwise, this layer will use an incomplete backdrop, limited to its
+    // render target and the blending result will be incorrect.
+    DCHECK(layer->uses_default_blend_mode() || IsRootLayer(layer) ||
+           !layer->parent()->render_target() ||
+           IsRootLayer(layer->parent()->render_target()) ||
+           layer->parent()->render_target()->is_root_for_isolated_group());
+
     render_surface->SetContentRect(clipped_content_rect);
 
     // The owning layer's screen_space_transform has a scale from content to