Replace kButton with kMark for PDF highlight

In CL:1934130 for PDF highlights kMark role replaced kButton role.
While retrieving static text node for a highlight node the check still
has a kButton as the expected role type. This CL replaces the kButton
role type with kMark role type for the same.

Bug: 1008775
Change-Id: Ia49eef757c71194173d35e71177e3fc5fb4a3fe5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1957747Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Ankit Kumar 🌪️ <ankk@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#722960}

components/pdf/renderer/pdf_accessibility_tree.cc

@@ -187,7 +187,7 @@ ui::AXNode* GetStaticTextNodeFromNode(ui::AXNode* node) {
   // Get the static text from the link node.
   if (node &&
       (node->data().role == ax::mojom::Role::kLink ||
-       node->data().role == ax::mojom::Role::kButton) &&
+       node->data().role == ax::mojom::Role::kMark) &&
       node->children().size() == 1) {
     static_node = node->children()[0];
   }