[web] Placeholders for rich paragraphs

lib/web_ui/dev/goldens_lock.yaml

@@ -1,2 +1,2 @@
 repository: https://github.com/flutter/goldens.git
-revision: 4946ab2de031c14d30502efcaf51220e0be4d1f1
+revision: 7529e9018b11c79334b99d1e7343fcd500c77b08

lib/web_ui/lib/src/engine/text/canvas_paragraph.dart

@@ -170,9 +170,7 @@ class CanvasParagraph implements EngineParagraph {
 
   @override
   List<ui.TextBox> getBoxesForPlaceholders() {
-    // TODO(mdebbar): After layout, placeholders positions should've been
-    // determined and can be used to compute their boxes.
-    return <ui.TextBox>[];
+    return _layoutService.getBoxesForPlaceholders();
   }
 
   // TODO(mdebbar): Check for child spans if any has styles that can't be drawn