a11y: Return if AXPlatformNodeAuraLinux doesn't have AtkObject

This CL fixes the assertion 'G_IS_OBJECT (object)' with an early
return if AXPlatformNodeAuraLinux doesn't have AtkObject as
GetOrCreateAtkObject() could return nullptr if AX mode is not
enabled.

It also updates |accessibility_mode_| from
BrowserAccessibilityStateImpl using AddAccessibilityModeFlags
instead of direct assigning since it should call
NotifyAddAXModeFlags() together when the mode is updated to sync
it with the mode from AXPlatformNodeAuraLinux.

Bug: 1061429
Change-Id: I0b4fa09d8d788608a403383c9e2c6123c618ccc9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2103997Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Julie Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#751904}

content/browser/accessibility/browser_accessibility_state_impl.cc

@@ -125,7 +125,7 @@ void BrowserAccessibilityStateImpl::ResetAccessibilityModeValue() {
   accessibility_mode_ = ui::AXMode();
   if (base::CommandLine::ForCurrentProcess()->HasSwitch(
           switches::kForceRendererAccessibility)) {
-    accessibility_mode_ = ui::kAXModeComplete;
+    AddAccessibilityModeFlags(ui::kAXModeComplete);
   }
 }
 

ui/accessibility/platform/ax_platform_node_auralinux.h

@@ -307,6 +307,8 @@ class AX_EXPORT AXPlatformNodeAuraLinux : public AXPlatformNodeBase {
 
   GType GetAccessibilityGType();
   AtkObject* CreateAtkObject();
+  // Get or Create AtkObject. Note that it could return nullptr except
+  // ax::mojom::Role::kApplication when the mode is not enabled.
   gfx::NativeViewAccessible GetOrCreateAtkObject();
   void DestroyAtkObjects();
   void AddRelationToSet(AtkRelationSet*,