[Code health] Make a comment in PaintWorkletProxyClient more clear

This CL has no behavior change.

Bug: None
Change-Id: I0efcac8a947d15e1f7b87f00d0bea0bf385c5fa9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1641517Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665678}

third_party/blink/renderer/modules/csspaint/paint_worklet_proxy_client.cc

@@ -104,8 +104,8 @@ void PaintWorkletProxyClient::RegisterCSSPaintDefinition(
       PaintWorklet::kNumGlobalScopes) {
     const Vector<AtomicString>& custom_properties =
         definition->CustomInvalidationProperties();
-    // AtomicString cannot be passed cross thread, which is why we need to make
-    // a String copy of it.
+    // Make a deep copy of the |custom_properties| into a Vector<String> so that
+    // CrossThreadCopier can pass that cross thread boundaries.
     Vector<String> passed_custom_properties;
     for (const auto& property : custom_properties)
       passed_custom_properties.push_back(property.GetString());