[Fuchsia] Add WebGL MSAA to the allowed list for WebEngine

Now webgl-antialiasing-mode and webgl-msaa-sample-count can be
specified in WebEngine config. This will allow to control MSAA support
when using WebGL.

Bug: 766360
Change-Id: If325bedefb31c1c960868563bcf223ba0bc88999
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2220865
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773101}

fuchsia/engine/context_provider_impl.cc

@@ -168,6 +168,8 @@ bool MaybeAddCommandLineArgsFromConfig(const base::Value& config,
       switches::kGpuRasterizationMSAASampleCount,
       switches::kMinHeightForGpuRasterTile,
       switches::kRendererProcessLimit,
+      switches::kWebglAntialiasingMode,
+      switches::kWebglMSAASampleCount,
   };
 
   for (const auto& arg : args->DictItems()) {