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

This introduces the plumbing required for custom paint.

PaintGeneratedImage - Holds onto a SkPicture for painting. This is
produced as the result of custom paint.

CSSPaintImageGeneratorImpl - Generates a PaintGeneratedImage based on a
LayoutObject and size. Invokes the custom paint callback inside
CSSPaintDefinition.

BUG=578252

Review URL: https://codereview.chromium.org/1866623002

Cr-Commit-Position: refs/heads/master@{#389838}
63 files changed