[Slimming Paint] Paint the inspector overlay with GraphicsLayer DisplayList.

Source/web/PageOverlay.h

Index: Source/web/PageOverlay.h
diff --git a/Source/web/PageOverlay.h b/Source/web/PageOverlay.h
index f55fbc1dde1a1ade0bf301cc1dd678bb90efb426..52bd4d65151c422ff0f5647a86479ac09cd24348 100644
--- a/Source/web/PageOverlay.h
+++ b/Source/web/PageOverlay.h
@@ -29,8 +29,11 @@
 #ifndef PageOverlay_h
 #define PageOverlay_h
 
+#include "platform/graphics/GraphicsLayer.h"
+#include "platform/graphics/GraphicsLayerClient.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/PassOwnPtr.h"
+#include "wtf/text/WTFString.h"
 
 namespace blink {
 
@@ -40,14 +43,32 @@ class GraphicsLayerClient;
 class WebPageOverlay;
 class WebViewImpl;
 
-class PageOverlay {
+// Manages a layer than is overlaid on a WebView's content.
+//
+// Clients can paint by providing either exactly one of the following:
+// - WebPageOverlay (which paints using a WebCanvas)
+// - PageOverlay::Painter (which paints using a GraphicsContext)
+//
+// In particular, if a client expects to paint web content (which, when
+// Slimming Paint is enabled, uses a DisplayItemList attached to
+// GraphicsContext), it should provide the latter.
+class PageOverlay : public GraphicsLayerClient {
 public:
+    class Painter {
+    public:
+        // The callee is responsible for saving the GraphicsContext,
+        // if it is required.
+        virtual void paintPageOverlay(GraphicsContext&) = 0;
+    };
+
     static PassOwnPtr<PageOverlay> create(WebViewImpl*, WebPageOverlay*);
+    static PassOwnPtr<PageOverlay> create(WebViewImpl*, Painter*);
 
     ~PageOverlay() { }
 
+    // Exactly one of these should be non-null.
     WebPageOverlay* overlay() const { return m_overlay; }
-    void setOverlay(WebPageOverlay* overlay) { m_overlay = overlay; }
+    Painter* overlayPainter() const { return m_overlayPainter; }
 
     int zOrder() const { return m_zOrder; }
     void setZOrder(int zOrder) { m_zOrder = zOrder; }
@@ -58,13 +79,19 @@ public:
 
     GraphicsLayer* graphicsLayer() const { return m_layer.get(); }
 
+    // GraphicsLayerClient implementation
+    void paintContents(const GraphicsLayer*, GraphicsContext&, GraphicsLayerPaintingPhase, const IntRect& inClip) override;
+    String debugName(const GraphicsLayer*) override;
+
 private:
     PageOverlay(WebViewImpl*, WebPageOverlay*);
+    PageOverlay(WebViewImpl*, Painter*);
+
     void invalidateWebFrame();
 
     WebViewImpl* m_viewImpl;
     WebPageOverlay* m_overlay;
-    OwnPtr<GraphicsLayerClient> m_layerClient;
+    Painter* m_overlayPainter;
     OwnPtr<GraphicsLayer> m_layer;
     int m_zOrder;
 };