Disable WebGL antialias in CanvasKit

We were accidentally turning on MSAA via the "antialias" context attrib,
then feeding the render target into Ganesh and saying it was non-MSAA.

This will cause rendering artifacts in Ganesh if MSAA is unknowingly
enabled when we try to do coverage-based AA.

Also, the WebGL spec does not give us control over the exact sample
count or even guarantee that "antialias" means MSAA, so I think it's
best to leave that flag disabled by default. If a client wants MSAA,
they can create their own offscreen surface and blit it into the main

Change-Id: I45f1596bfe9258963cff4b5d0a3921c5ba43145f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/291029
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
2 files changed