Implement PaintRenderingContext2D off BaseRenderingContext2D

This exposes the subset of the API which we'll need for csspaint.
https://drafts.css-houdini.org/css-paint-api/#paintrenderingcontext2d

Most of the code is just a straight pass-through to the BaseRenderingContext2D.

BUG=578252

Review URL: https://codereview.chromium.org/1778413003

Cr-Commit-Position: refs/heads/master@{#382788}
6 files changed