Remove unused headers from slot_assignment.cc

Change-Id: I8cfd1db4b3b48b69c478a0bc5a60402e035272eb
Reviewed-on: https://chromium-review.googlesource.com/1162085Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580810}

third_party/blink/renderer/core/dom/slot_assignment.cc

@@ -9,12 +9,10 @@
 #include "third_party/blink/renderer/core/dom/node_traversal.h"
 #include "third_party/blink/renderer/core/dom/shadow_root.h"
 #include "third_party/blink/renderer/core/dom/slot_assignment_engine.h"
-#include "third_party/blink/renderer/core/dom/v0_insertion_point.h"
 #include "third_party/blink/renderer/core/html/forms/html_opt_group_element.h"
 #include "third_party/blink/renderer/core/html/forms/html_select_element.h"
 #include "third_party/blink/renderer/core/html/html_details_element.h"
 #include "third_party/blink/renderer/core/html/html_slot_element.h"
-#include "third_party/blink/renderer/core/html_names.h"
 
 namespace blink {