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

This is a reland of 4b053da1

Original change's description:
> [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}

Bug: 766360
Change-Id: I763044e3419652f2a5682432ad6f04f5718c658d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2218780
Auto-Submit: Wez <wez@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773197}

content/public/common/content_switches.h

@@ -239,8 +239,8 @@ CONTENT_EXPORT extern const char kUtilitySubType[];
 CONTENT_EXPORT extern const char kV8CacheOptions[];
 CONTENT_EXPORT extern const char kValidateInputEventStream[];
 CONTENT_EXPORT extern const char kWaitForDebuggerChildren[];
-extern const char kWebglAntialiasingMode[];
-extern const char kWebglMSAASampleCount[];
+CONTENT_EXPORT extern const char kWebglAntialiasingMode[];
+CONTENT_EXPORT extern const char kWebglMSAASampleCount[];
 
 CONTENT_EXPORT extern const char kWebOtpBackend[];
 CONTENT_EXPORT extern const char kWebOtpBackendSmsVerification[];

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()) {