[Fuchsia] Add MSAA flags in the list of allowed switches for WebEngine

Added --canvas-msaa-sample-count and --gpu-rasterization-msaa-sample-count
in the list of switches allowed in the WebEngine config. These flags are
useful to turn off MSAA, which may be required to save memory.

Bug: fuchsia:43693
Change-Id: Ib519cc7bede46a5ff7879d1a9678fbf88e33de2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2040200
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738719}

fuchsia/engine/context_provider_impl.cc

@@ -152,6 +152,7 @@ bool MaybeAddCommandLineArgsFromConfig(const base::Value& config,
     return true;
 
   static const base::StringPiece kAllowedArgs[] = {
+      switches::kAcceleratedCanvas2dMSAASampleCount,
       switches::kDisableFeatures,
       switches::kDisableGpuWatchdog,
       switches::kEnableFeatures,
@@ -160,6 +161,7 @@ bool MaybeAddCommandLineArgsFromConfig(const base::Value& config,
       switches::kForceGpuMemAvailableMb,
       switches::kForceGpuMemDiscardableLimitMb,
       switches::kForceMaxTextureSize,
+      switches::kGpuRasterizationMSAASampleCount,
       switches::kMinHeightForGpuRasterTile,
       switches::kRendererProcessLimit,
   };