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
canvas.

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>

modules/canvaskit/CHANGELOG.md

@@ -13,6 +13,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
  - `CanvasKitInit(...)` now directly returns a Promise. As such, `CanvasKitInit(...).ready()`
    has been removed.
 
+### Fixed
+ - WebGL context is no longer created with "antialias" flag. Using "antialias" caused poor AA
+   quality in Ganesh when trying to do coverage-based AA with MSAA unknowingly enabled. It also
+   reduced performance.
+
 ## [0.15.0] - 2020-05-14
 
 ### Added

modules/canvaskit/gpu.js

@@ -18,7 +18,7 @@
           'alpha': get(attrs, 'alpha', 1),
           'depth': get(attrs, 'depth', 1),
           'stencil': get(attrs, 'stencil', 8),
-          'antialias': get(attrs, 'antialias', 1),
+          'antialias': get(attrs, 'antialias', 0),
           'premultipliedAlpha': get(attrs, 'premultipliedAlpha', 1),
           'preserveDrawingBuffer': get(attrs, 'preserveDrawingBuffer', 0),
           'preferLowPowerToHighPerformance': get(attrs, 'preferLowPowerToHighPerformance', 0),