Update AX role mapping for summary to AXButton on Mac

ui/accessibility/platform/ax_platform_node_mac.mm

Index: ui/accessibility/platform/ax_platform_node_mac.mm
diff --git a/ui/accessibility/platform/ax_platform_node_mac.mm b/ui/accessibility/platform/ax_platform_node_mac.mm
index 240240e6180bc11f40e56de4fa36fdafd9e65b81..6c0deebff19dc45fe199e2880290ac437aa37261 100644
--- a/ui/accessibility/platform/ax_platform_node_mac.mm
+++ b/ui/accessibility/platform/ax_platform_node_mac.mm
@@ -66,7 +66,11 @@ RoleMap BuildRoleMap() {
       {ui::AX_ROLE_DIALOG, NSAccessibilityGroupRole},
       {ui::AX_ROLE_DETAILS, NSAccessibilityGroupRole},
       {ui::AX_ROLE_DIRECTORY, NSAccessibilityListRole},
-      {ui::AX_ROLE_DISCLOSURE_TRIANGLE, NSAccessibilityDisclosureTriangleRole},
+      // If Mac supports AXExpandedChanged event with
+      // NSAccessibilityDisclosureTriangleRole, We should update
+      // AX_ROLE_DISCLOSURE_TRIANGLE mapping to
+      // NSAccessibilityDisclosureTriangleRole. http://crbug.com/558324
+      {ui::AX_ROLE_DISCLOSURE_TRIANGLE, NSAccessibilityButtonRole},
       {ui::AX_ROLE_DOCUMENT, NSAccessibilityGroupRole},
       {ui::AX_ROLE_EMBEDDED_OBJECT, NSAccessibilityGroupRole},
       {ui::AX_ROLE_FIGCAPTION, NSAccessibilityGroupRole},