PaintWorklet: Move paintWorklet from "window" to "CSS"

Before this CL, "paintWorklet" attribute is defined on "Window" interface, but
that should be defined on "CSS" interface[1].

According to the spec discussion[2], "CSS" will be changed from "interface" to
"namespace", but "namespace" is not supported yet in Blink. As a stopgap, this
CL changes "paintWorklet" to a static attribute and moves it to "CSS" interface
to emulate an attribute-on-namespace.

[1] https://drafts.css-houdini.org/css-paint-api-1/#dom-css-paintworklet
[2] w3c/css-houdini-drafts#410

Bug: 719303
Change-Id: I01f371f0d4f9a193352d1ede1fe28632d52cf312
Reviewed-on: https://chromium-review.googlesource.com/630899
Commit-Queue: Hiroki Nakagawa 
Reviewed-by: Xida Chen 
Reviewed-by: Ian Kilpatrick 
Reviewed-by: Kentaro Hara 
WPT-Export-Revision: ff3957a21bc1bc8bf5df4e44f29832e424dfa49e

Author: nhiroki

common/css-paint-tests.js

@@ -2,11 +2,11 @@
 // requestAnimationFrame. In the second frame, we take a screenshot, that makes
 // sure that we already have a full frame.
 function importPaintWorkletAndTerminateTestAfterAsyncPaint(code) {
-    if (typeof paintWorklet == "undefined") {
+    if (typeof CSS.paintWorklet == "undefined") {
         takeScreenshot();
     } else {
         var blob = new Blob([code], {type: 'text/javascript'});
-        paintWorklet.addModule(URL.createObjectURL(blob)).then(function() {
+        CSS.paintWorklet.addModule(URL.createObjectURL(blob)).then(function() {
             requestAnimationFrame(function() {
                 requestAnimationFrame(function() {
                     takeScreenshot();

css-paint-api/valid-image-after-load.html

@@ -30,7 +30,7 @@
 var blob = new Blob([document.getElementById('code').textContent],
                     {type: 'text/javascript'});
 var frame_cnt = 0;
-paintWorklet.addModule(URL.createObjectURL(blob)).then(function() {
+CSS.paintWorklet.addModule(URL.createObjectURL(blob)).then(function() {
     var el = document.getElementById('output');
     el.style.backgroundImage = 'paint(green)';
     requestAnimationFrame(function() {

css-paint-api/valid-image-before-load.html

@@ -33,7 +33,7 @@
 var blob = new Blob([document.getElementById('code').textContent],
                     {type: 'text/javascript'});
 var frame_cnt = 0;
-paintWorklet.addModule(URL.createObjectURL(blob)).then(function() {
+CSS.paintWorklet.addModule(URL.createObjectURL(blob)).then(function() {
     requestAnimationFrame(function() {
         takeScreenshot(frame_cnt);
     });