Oilpan: ImageObserver needs to be a GC mixin.

third_party/WebKit/Source/core/fetch/ImageResource.h

Index: third_party/WebKit/Source/core/fetch/ImageResource.h
diff --git a/third_party/WebKit/Source/core/fetch/ImageResource.h b/third_party/WebKit/Source/core/fetch/ImageResource.h
index 277ea80986ddd9f8f2dc60529d92a79ec169d8a2..fd108b5944341fddaab04590d0749fba2901cc18 100644
--- a/third_party/WebKit/Source/core/fetch/ImageResource.h
+++ b/third_party/WebKit/Source/core/fetch/ImageResource.h
@@ -44,7 +44,7 @@ class SecurityOrigin;
 
 class CORE_EXPORT ImageResource final : public Resource, public ImageObserver {
     friend class MemoryCache;
-
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(ImageResource);
 public:
     using ClientType = ImageResourceClient;
 
@@ -121,6 +121,8 @@ public:
     void animationAdvanced(const blink::Image*) override;
     void changedInRect(const blink::Image*, const IntRect&) override;
 
+    DECLARE_VIRTUAL_TRACE();
+
 protected:
     bool isSafeToUnlock() const override;
     void destroyDecodedDataIfPossible() override;