Introduce new API to test mixed state of checkboxes.

content/shell/renderer/test_runner/web_ax_object_proxy.cc

Index: content/shell/renderer/test_runner/web_ax_object_proxy.cc
diff --git a/content/shell/renderer/test_runner/web_ax_object_proxy.cc b/content/shell/renderer/test_runner/web_ax_object_proxy.cc
index 2ffc21f0fe98c23030debb9174e72235758a5a06..6292cfcb01183d1b32d19c12f57a5c200adcd6eb 100644
--- a/content/shell/renderer/test_runner/web_ax_object_proxy.cc
+++ b/content/shell/renderer/test_runner/web_ax_object_proxy.cc
@@ -503,6 +503,7 @@ WebAXObjectProxy::GetObjectTemplateBuilder(v8::Isolate* isolate) {
       .SetProperty("rowCount", &WebAXObjectProxy::RowCount)
       .SetProperty("columnCount", &WebAXObjectProxy::ColumnCount)
       .SetProperty("isClickable", &WebAXObjectProxy::IsClickable)
+      .SetProperty("isButtonStateMixed", &WebAXObjectProxy::IsButtonStateMixed)
       .SetMethod("allAttributes", &WebAXObjectProxy::AllAttributes)
       .SetMethod("attributesOfChildren",
                  &WebAXObjectProxy::AttributesOfChildren)
@@ -797,6 +798,11 @@ bool WebAXObjectProxy::IsClickable() {
   return accessibility_object_.isClickable();
 }
 
+bool WebAXObjectProxy::IsButtonStateMixed() {
+  accessibility_object_.updateLayoutAndCheckValidity();
+  return accessibility_object_.isButtonStateMixed();
+}
+
 std::string WebAXObjectProxy::AllAttributes() {
   accessibility_object_.updateLayoutAndCheckValidity();
   return GetAttributes(accessibility_object_);