Remove display item scope

third_party/WebKit/Source/platform/graphics/paint/README.md

 # Platform paint code
 
 This directory contains the implementation of display lists and display
 list-based painting, except for code which requires knowledge of `core/`
 concepts, such as DOM elements and layout objects.
 
 This code is owned by the [paint team][paint-team-site].
 
 Slimming Paint v2 is currently being implemented. Unlike Slimming Paint v1, SPv2
 represents its paint artifact not as a flat display list, but as a list of
@@ skipping 100 matching lines @@
 exists, as well as effects other than opacity.
 ***
 
 ## Display items
 
 A display item is the smallest unit of a display list in Blink. Each display
 item is identified by an ID consisting of:
 
 * an opaque pointer to the *display item client* that produced it
 * a type (from the `DisplayItem::Type` enum)
-* a scope number
-
-*** aside
-TODO(jbroman): Explain scope numbers.
-***
 
 In practice, display item clients are generally subclasses of `LayoutObject`,
 but can be other Blink objects which get painted, such as inline boxes and drag
 images.
 
 *** note
 It is illegal for there to be two drawings with the same ID in a display item
-list.
+list, except for drawings that are marked uncacheable
+(see [DisplayItemCacheSkipper](DisplayItemCacheSkipper.h)).
 ***
 
 Generally, clients of this code should use stack-allocated recorder classes to
 emit display items to a `PaintController` (using `GraphicsContext`).
 
 ### Standalone display items
 
 #### [CachedDisplayItem](CachedDisplayItem.h)
 
 See [Display item caching](../../../core/paint/README.md#paint-result-caching).
@@ skipping 87 matching lines @@
 The [`PaintArtifactCompositor`](PaintArtifactCompositor.h) is responsible for
 consuming the `PaintArtifact` produced by the `PaintController`, and converting
 it into a form suitable for the compositor to consume.
 
 At present, `PaintArtifactCompositor` creates a cc layer tree, with one layer
 for each paint chunk. In the future, it is expected that we will use heuristics
 to combine paint chunks into a smaller number of layers.
 
 The owner of the `PaintArtifactCompositor` (e.g. `WebView`) can then attach its
 root layer to the overall layer hierarchy to be displayed to the user.