[LayoutNG] Remove a stale TODO

Tbr: eae@chromium.org
NoTry: True
Change-Id: I8811ef4a85c350663e280543f76a950abae858a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1505901Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638309}

third_party/blink/renderer/core/layout/ng/inline/ng_offset_mapping_builder.h

@@ -20,7 +20,6 @@ class LayoutObject;
 // mapping. It holds an offset mapping, and provides APIs to modify the mapping
 // step by step until the construction is finished.
 // Design doc: https://goo.gl/CJbxky
-// TODO(xiaochengh): Change the mock implemetation to a real one.
 class CORE_EXPORT NGOffsetMappingBuilder {
   STACK_ALLOCATED();