Bug 1769283 - Unable to set MSAA samples in GeckoView. r=agi

GeckoView is still using the deleted gl.msaa-level setting. As the rest of the engine does not use it,
modifying it basically became a noop. Replacing it by webgl.msaa-samples does the trick and allows
GeckoView clients to modify the antialiasing settings in WebGL.

Differential Revision: https://phabricator.services.mozilla.com/D146322

mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoRuntimeSettings.java

@@ -483,7 +483,7 @@ public final class GeckoRuntimeSettings extends RuntimeSettings {
   /* package */ final Pref<Boolean> mInputAutoZoom = new Pref<>("formhelper.autozoom", true);
   /* package */ final Pref<Boolean> mDoubleTapZooming =
       new Pref<>("apz.allow_double_tap_zooming", true);
-  /* package */ final Pref<Integer> mGlMsaaLevel = new Pref<>("gl.msaa-level", 0);
+  /* package */ final Pref<Integer> mGlMsaaLevel = new Pref<>("webgl.msaa-samples", 4);
   /* package */ final Pref<Boolean> mTelemetryEnabled =
       new Pref<>("toolkit.telemetry.geckoview.streaming", false);
   /* package */ final Pref<String> mGeckoViewLogLevel = new Pref<>("geckoview.logging", "Debug");