Intent to prototype: CanvasRenderingContext2D.createConicGradient.

[Emilio Cobos Álvarez]

Sending this on behalf of the contributor that submitted the patch.

Summary: Implement a method analogous to createLinearGradient and
createRadialGradient for canvas.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1627014

Standard: https://github.com/whatwg/html/issues/5431

Platform coverage: All

Preference: canvas.createConicGradient.enabled

DevTools bug: N/A, I think

Other browsers:

* Chromium: Implemented behind flag, pending some minor spec issues
being sorted out I think.
* WebKit: Supportive
(https://github.com/whatwg/html/issues/5431#issuecomment-641097348)

web-platform-tests:
html/canvas/element/fill-and-stroke-styles/2d.gradient.conic.html

-- Emilio