[web] dont look up webgl params if no GPU is available (flutter/engine#38948)

engine/src/flutter/lib/web_ui/lib/src/engine/canvaskit/surface.dart

@@ -336,9 +336,6 @@ class Surface {
           majorVersion: webGLVersion.toDouble(),
         ),
       ).toInt();
-      if (_sampleCount == -1 || _stencilBits == -1) {
-        _initWebglParams();
-      }
 
       _glContext = glContext;
 
@@ -348,6 +345,9 @@ class Surface {
           throw CanvasKitError('Failed to initialize CanvasKit. '
               'CanvasKit.MakeGrContext returned null.');
         }
+        if (_sampleCount == -1 || _stencilBits == -1) {
+          _initWebglParams();
+        }
         // Set the cache byte limit for this grContext, if not specified it will
         // use CanvasKit's default.
         _syncCacheBytes();