Correct include guard in aom_context_ax_tree.h

A trailing "}" was not included in the include guard so if
included multiple time, you would get "extraneous }" errors.

Change-Id: I32582e928220ee9428acbefa9a4833d9d36d2eec
Reviewed-on: https://chromium-review.googlesource.com/888581Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#532172}

content/renderer/accessibility/aom_content_ax_tree.h

@@ -34,6 +34,6 @@ class AomContentAxTree : public blink::WebComputedAXTree {
   DISALLOW_COPY_AND_ASSIGN(AomContentAxTree);
 };
 
-#endif  // CONTENT_RENDERER_ACCESSIBILITY_AOM_CONTENT_AX_TREE_H_
-
 }  // namespace content
+
+#endif  // CONTENT_RENDERER_ACCESSIBILITY_AOM_CONTENT_AX_TREE_H_