Move iframe and plugin compositing triggers into additionalCompositingReasons

Source/core/rendering/RenderPart.h

Index: Source/core/rendering/RenderPart.h
diff --git a/Source/core/rendering/RenderPart.h b/Source/core/rendering/RenderPart.h
index 707fde3d463aefca343cb63d4041740837610b57..2376e45941c348436c75d04491f301f17fb50f5f 100644
--- a/Source/core/rendering/RenderPart.h
+++ b/Source/core/rendering/RenderPart.h
@@ -48,6 +48,8 @@ protected:
 private:
     virtual bool isRenderPart() const OVERRIDE FINAL { return true; }
     virtual const char* renderName() const OVERRIDE { return "RenderPart"; }
+
+    virtual CompositingReasons additionalCompositingReasons(CompositingTriggerFlags) const OVERRIDE;
 };
 
 DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderPart, isRenderPart());