Remove unnecessary `?`s from web_ui. (#21444)

* Remove unnecessary `?`s from web_ui.

In https://dart-review.googlesource.com/c/sdk/+/163841, dart's flow
analysis is being changed so that an assignment like this:

    T? x = <non-nullable value>;

causes `x` to be immediately promoted to type `T`.  This exposed two
instances in which a variable was unnecessarily given a nullable type.
To avoid warnings, we need to fix these two types before
https://dart-review.googlesource.com/c/sdk/+/163841 can land.

lib/web_ui/lib/src/engine/bitmap_canvas.dart

@@ -608,11 +608,11 @@ class BitmapCanvas extends EngineCanvas {
     double x,
     double y,
   ) {
-    html.CanvasRenderingContext2D? ctx = _canvasPool.context;
+    html.CanvasRenderingContext2D ctx = _canvasPool.context;
     x += line.left;
     final double? letterSpacing = style.letterSpacing;
     if (letterSpacing == null || letterSpacing == 0.0) {
-      ctx!.fillText(line.displayText!, x, y);
+      ctx.fillText(line.displayText!, x, y);
     } else {
       // When letter-spacing is set, we go through a more expensive code path
       // that renders each character separately with the correct spacing
@@ -627,7 +627,7 @@ class BitmapCanvas extends EngineCanvas {
       final int len = line.displayText!.length;
       for (int i = 0; i < len; i++) {
         final String char = line.displayText![i];
-        ctx!.fillText(char, x, y);
+        ctx.fillText(char, x, y);
         x += letterSpacing + ctx.measureText(char).width!;
       }
     }

lib/web_ui/lib/src/engine/canvaskit/canvaskit_api.dart

@@ -84,7 +84,7 @@ class CanvasKit {
   external int GetWebGLContext(
       html.CanvasElement canvas, SkWebGLContextOptions options);
   external SkGrContext MakeGrContext(int glContext);
-  external SkSurface MakeOnScreenGLSurface(
+  external SkSurface? MakeOnScreenGLSurface(
     SkGrContext grContext,
     int width,
     int height,