Don't remove empty display item pairs that draw content

Source/platform/graphics/paint/DrawingDisplayItem.h

Index: Source/platform/graphics/paint/DrawingDisplayItem.h
diff --git a/Source/platform/graphics/paint/DrawingDisplayItem.h b/Source/platform/graphics/paint/DrawingDisplayItem.h
index 8aff94a9bab414cd9429b9abc255a1ad79236fb0..d98f0161b3eb0640e55f89bc5a6a9b25f3f6e77f 100644
--- a/Source/platform/graphics/paint/DrawingDisplayItem.h
+++ b/Source/platform/graphics/paint/DrawingDisplayItem.h
@@ -23,6 +23,8 @@ public:
     virtual void replay(GraphicsContext*);
     virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override;
 
+    virtual bool drawsContent() const override { return true; }

[Stephen Chennney, 2015/02/19 16:13:27]

This should check the picture contents. approximateOpCount == 0 implies a
strictly empty picture. A picture with non-zero op-count may also draw nothing,
but that's harder to assess so not useful in this case.

[ajuma, 2015/02/19 17:36:51]

Done.

+
     PassRefPtr<const SkPicture> picture() const { return m_picture; }
 
 protected: