[a11y] Set AX mode for AtkUtilAuraLinuxTest

This CL adds AX mode configuration for AtkUtilAuraLinuxTest that
needs to access ATK object since we don't create ATK object if
the mode is not enabled.

On build bots, it's not failed as it works with other tests in
the same process and the configuration is based on a process. But
if it runs individually, it's not passed. So, this CL adds the
configuration to the test.

AX-Relnotes: n/a.
Bug: 1100797
Change-Id: I6def01fef97c3f8b00b4130197c321d3625ed313
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274624
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783981}

ui/accessibility/platform/atk_util_auralinux_unittest.cc

@@ -17,6 +17,7 @@ namespace ui {
 class AtkUtilAuraLinuxTest : public AXPlatformNodeTest {
  public:
   AtkUtilAuraLinuxTest() {
+    AXPlatformNode::NotifyAddAXModeFlags(kAXModeComplete);
     // We need to create a platform node in order to install it as the root
     // ATK node. The ATK bridge will complain if we try to use it without a
     // root node installed.