Rename AddPaintRegion to AddExistingPaintRegion

This overload is used when reusing paint region from retained layers and name should reflect it.

Author: knopp

flow/diff_context.cc

@@ -92,7 +92,10 @@ void DiffContext::AddPaintRegion(const SkRect& rect) {
   }
 }
 
-void DiffContext::AddPaintRegion(const PaintRegion& region) {
+void DiffContext::AddExistingPaintRegion(const PaintRegion& region) {
+  // Adding paint region for retained layer implies that current subtree is not
+  // dirty
+  FML_DCHECK(!IsSubtreeDirty());
   FML_DCHECK(region.is_valid());
   rects_->insert(rects_->end(), region.begin(), region.end());
 }

flow/diff_context.h

@@ -134,8 +134,8 @@ class DiffContext {
   // Add paint region for layer; rect is in "local" (layer) coordinates
   void AddPaintRegion(const SkRect& rect);
 
-  // Add entire paint region for current subtree
-  void AddPaintRegion(const PaintRegion& region);
+  // Add entire paint region of retained layer for current subtree
+  void AddExistingPaintRegion(const PaintRegion& region);
 
   // The idea of readback region is that if any part of the readback region
   // needs to be repainted, then the whole readback region must be repainted;

flow/layers/container_layer.cc

@@ -86,7 +86,7 @@ void ContainerLayer::DiffChildren(DiffContext* context,
         // previous frame; We can only do this if there is no readback in the
         // subtree. Layers that do readback must be able to register readback
         // inside Diff
-        context->AddPaintRegion(paint_region);
+        context->AddExistingPaintRegion(paint_region);
 
         // While we don't need to diff retained layers, we still need to
         // associate their paint region with current layer tree so that we can