[canvaskit] Add .ready() which is a real promise

Prior to this, CanvasKit was unintentionally a thenable, which means if
it was used to resolve a Promise, certain logic would be called which
would put it into an infinite loop. By adding a .ready() which returns
a proper Promise and removes the .then(), this should make the CanvasKit
libary safer.

For now, the .then() still exists unless .ready() is called. .then will
be removed unconditionally in 0.4.0 which will be out after all known
clients (e.g. jsfiddle.skia.org) are changed.

See: https://github.com/kripken/emscripten/issues/5820

Bug: skia:
Change-Id: Ie4093f6b0ce03070ef737941693b06dfff93f61c
Reviewed-on: https://skia-review.googlesource.com/c/181177
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
11 files changed