Implement HTMLCanvasElement's transferControlToOffscreen API

[jdm]

The canvas API supports taking an existing HTMLCanvasElement, calling the transferControlToOffscreen method and obtaining a new OffscreenCanvas which wraps the existing canvas state. The existing canvas element changes its mode to the placeholder mode, and its content is only updated when calling the commit method of the new OffscreenCanvas.

WebIDL:

• https://html.spec.whatwg.org/multipage/canvas.html#the-canvas-element
• https://html.spec.whatwg.org/multipage/canvas.html#the-offscreen-2d-rendering-context
  Spec:
• https://html.spec.whatwg.org/multipage/canvas.html#dom-canvas-transfercontroltooffscreen
• https://html.spec.whatwg.org/multipage/canvas.html#offscreencontext2d-commit

Code:

• components/script/dom/webidls/HTMLCanvasElement.webidl
• components/script/dom/webidls/OffscreenCanvasRenderingContext2D.webidl
• components/script/dom/htmlcanvaselement.rs
• components/script/dom/offscreenrenderingcontext2d.rs

Tests: ./mach test-wpt tests/wpt/web-platform-tests/offscreen-canvas/the-canvas-element

[gterzian]

The comment at #24272 (comment) applies to this as well.