Introduce PaintWorkletGlobalScope, rename renderWorklet attribute to paintWorklet.

Was decided at Sydney F2F to split everything out into it's own worklet at the moment.
This may get revisited at some point in time.
https://drafts.css-houdini.org/css-paint-api/#dom-window-paintworklet

Introduces PaintWorkletGlobalScope with stub function for implementing registerPaint().
https://drafts.css-houdini.org/css-paint-api/#paintworkletglobalscope

BUG=578252

Review URL: https://codereview.chromium.org/1818223002

Cr-Commit-Position: refs/heads/master@{#382744}
24 files changed