Trivial: Fixing comment on ax_node_data Add*Attribute methods.

Comment had a 'not' which was likely a mistake, the Add*Attribute methods all
remove the attribute if it already exists before adding.

R=aboxhall,dmazzoni

Change-Id: Iaf0bc259ba8c6e9f64f4b443935ff01979ca893d
Reviewed-on: https://chromium-review.googlesource.com/c/1485613
Auto-Submit: Chris Hall <chrishall@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635159}

ui/accessibility/ax_node_data.h

@@ -97,8 +97,8 @@ struct AX_EXPORT AXNodeData {
   //
   // Setting accessibility attributes.
   //
-  // Replaces an attribute if not present. This is safer than crashing via a
-  // DCHECK or doing nothing, because most likely that's what the caller would
+  // Replaces an attribute if present. This is safer than crashing via a DCHECK
+  // or doing nothing, because most likely replacing is what the caller would
   // have wanted or what existing code already assumes.
   //