[canvaskit] Allow providing webgl context and canvas element

Maybe one day we'll want to expose makeWebGLContext too, thus
it takes in the attrs as a configurable parameter.

Bug: skia:
Change-Id: Idb38f932b5ea6b364d823283b262e8508bff07c4
Reviewed-on: https://skia-review.googlesource.com/c/180776
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
4 files changed