Hook up CSSPaintValue::image to CSS Paint API callback.

third_party/WebKit/Source/modules/csspaint/README.md

+# CSS Paint API
+
+This directory contains the implementation of the CSS Paint API.
+
+See [CSS Paint API](https://drafts.css-houdini.org/css-paint-api/) for the web exposed APIs this
+implements.
+
+## Implementation
+
+### [CSSPaintDefinition](CSSPaintDefinition.h)
+
+Represents a class registered by the author through `PaintWorkletGlobalScope#registerPaint`.
+Specifically this class holds onto the javascript constructor and paint functions of the class via
+persistent handles. This class keeps these functions alive so they don't get garbage collected.
+
+The `CSSPaintDefinition` also holds onto an instance of the paint class via a weak persistent
+handle. The actual instance of the paint class is kept alive by `CSSPaintImageGeneratorImpl`. This 
+is so the instance can be garbage collected when not in use anymore.
+
+The `PaintWorkletGlobalScope` has a map of paint `name` to `CSSPaintDefinition`.
+
+### [CSSPaintImageGenerator][generator] and [CSSPaintImageGeneratorImpl][generator-impl]
+
+`CSSPaintImageGenerator` represents the interface from which the `CSSPaintValue` can generate
+`Image`s from. This is done via the `CSSPaintImageGenerator#paint` method. Each `CSSPaintValue`
+owns a separate instance of `CSSPaintImageGenerator`.
+
+`CSSPaintImageGeneratorImpl` is the implementation which lives in `modules/csspaint`. (We have this
+interface / implementation split as `core/` cannot depend on `modules/`).
+
+When created the generator will access paint worklet and lookup it's corresponding
+`CSSPaintDefinition` via `PaintWorkletGlobalScope#findDefinition`. It will also keep a persistent
+handle to the javascript paint class instance to keep this alive.
+
+If the paint worklet does not have a `CSSPaintDefinition` matching the paint `name` the
+`CSSPaintImageGeneratorImpl` is placed in a "pending" map. Once a paint class with `name` is
+registered the generator is notified so it can invalidate an display the correct image.
+
+[generator]: ../../core/css/CSSPaintImageGenerator.h
+[generator-impl]: CSSPaintImageGeneratorImpl.h
+[paint-value]: ../../core/css/CSSPaintValue.h
+
+### Generating a [PaintGeneratedImage](../../platform/graphics/PaintGeneratedImage.h)
+
+`PaintGeneratedImage` is a `Image` which just paints a single `SkPicture`.
+
+A `CSSPaintValue` can generate an image from the method `CSSPaintImageGenerator#paint`. This method
+calls through to `CSSPaintDefinition#paint` which actually invokes the javascript paint method.
+This method returns the `PaintGeneratedImage`.
+
+The `SkPicture` is produced from a `RecordingImageBufferSurface`.
+
+## Testing
+
+Tests live [here](../../../LayoutTests/csspaint/).
+