[a11y] Remove comment on StringAttribute::kClassName

Remove the Native / Android comment on the StringAttribute kClassName
since that value is no longer populated by Android and is used on all
platforms.

Change-Id: I361912e66c8c2c0b4cd136a02412d135ac0180bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404408Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805903}

ui/accessibility/ax_enums.mojom

@@ -502,7 +502,7 @@ enum StringAttribute {
   kAriaInvalidValue,
   kAutoComplete,
   kChildTreeId,
-  kClassName,  // Native / Android
+  kClassName,
   kContainerLiveRelevant,
   kContainerLiveStatus,
   kDescription,