Make DisplayItemClient an interface

third_party/WebKit/Source/core/svg/graphics/SVGImage.h

Index: third_party/WebKit/Source/core/svg/graphics/SVGImage.h
diff --git a/third_party/WebKit/Source/core/svg/graphics/SVGImage.h b/third_party/WebKit/Source/core/svg/graphics/SVGImage.h
index 416f2658dfd8b4c4cfd2f265aa2727819a9952ce..1b5008cc4b1a4dcf9a830f6ca0557e6fb94673b1 100644
--- a/third_party/WebKit/Source/core/svg/graphics/SVGImage.h
+++ b/third_party/WebKit/Source/core/svg/graphics/SVGImage.h
@@ -42,7 +42,7 @@ class LayoutBox;
 class SVGImageChromeClient;
 class SVGImageForContainer;
 
-class SVGImage final : public Image {
+class SVGImage final : public Image, public DisplayItemClient {

[pdr., 2015/12/05 21:39:19]

This could be problematic because a single SVGImage can be shared by multiple
documents since it lives in the memorycache. For example, if two tabs both
reference happy.svg, and the two tabs happen to be in the same process, they
will share one SVGImage instance.

It looks like SVGImage is only a superclass of DisplayItemClient for two
SkPictureBuilder calls. Can we switch those to use a null DisplayItemClient* so
SVGImage doesn't need to be a DisplayItemClient?

[Xianzhu, 2015/12/07 17:14:00]

If you meant the problem that the invalidation bits would be modified by
different PaintControllers, I think it can be avoided by adding a
CacheableDisplayItemClient type which contains methods for accessing
invalidation bits, while SVGImage can be still a normal non-cacheable
DisplayItemClient.     
I don't quite understand the use cases you mentioned. Can you provide links to
the code?

The current patch requires non-null DisplayItemClients. Null DisplayItemClient
seems to increase complexity and eliminate the ability to track display item
sources for debugging.

 public:
     static PassRefPtr<SVGImage> create(ImageObserver* observer)
     {
@@ -77,8 +77,7 @@ public:
 
     void updateUseCounters(Document&) const;
 
-    DisplayItemClient displayItemClient() const { return toDisplayItemClient(this); }
-    String debugName() const { return "SVGImage"; }
+    String debugName() const final { return "SVGImage"; }
 
 private:
     friend class AXLayoutObject;