Remove unused headers from slot_assignment.h

Change-Id: Icd3665e37131c31c584a97355b3f74d76b0f34ff
Reviewed-on: https://chromium-review.googlesource.com/1162084Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580812}

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

@@ -7,10 +7,7 @@
 
 #include "third_party/blink/renderer/core/dom/tree_ordered_map.h"
 #include "third_party/blink/renderer/platform/heap/handle.h"
-#include "third_party/blink/renderer/platform/runtime_enabled_features.h"
-#include "third_party/blink/renderer/platform/wtf/hash_map.h"
 #include "third_party/blink/renderer/platform/wtf/text/atomic_string.h"
-#include "third_party/blink/renderer/platform/wtf/text/atomic_string_hash.h"
 
 namespace blink {