Intent to implement: canvas getTransform() and setTransform(DOMMatrixInit) methods.

[Thomas Wisniewski]

The canvas draft spec used to have a currentTransform attribute on 2D
canvas context objects, which was implemented by Chrome (behind a
runtime flag). We also ship our own similar (but non-standard)
mozGetTransform and mozSetTransform methods.

The draft spec has since changed to add a getTransform method
returning a DOMMatrix, and a setTransform method that accepts a new
DOMMatrixInit dictionary.

It seems that the Chrome team intends to support these new methods
instead of currentTransform, so I would like to implement them as
well, and help push the spec forward.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=928150
Proposed preference behind which to implement:
canvas.transform_extensions.enabled
Draft spec: https://html.spec.whatwg.org/multipage/scripting.html#current-transformation-matrix
Chrome bug: https://bugs.chromium.org/p/chromium/issues/detail?id=637940