[Squad] Don't create PseudoElementData for setting a nullptr.

SetPseudoElement created a PseudoElementData even if the input was a
nullptr. Early out for null input to avoid unnecessary allocation.

Bug: 836126
Change-Id: If21c9ff8db2f20427b32f124ea0b129ebbebfe80
Reviewed-on: https://chromium-review.googlesource.com/1109962Reviewed-by: Anders Ruud <andruud@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569625}

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

@@ -241,8 +241,11 @@ inline void ElementRareData::ClearPseudoElements() {
 
 inline void ElementRareData::SetPseudoElement(PseudoId pseudo_id,
                                               PseudoElement* element) {
-  if (!pseudo_element_data_)
+  if (!pseudo_element_data_) {
+    if (!element)
+      return;
     pseudo_element_data_ = PseudoElementData::Create();
+  }
   pseudo_element_data_->SetPseudoElement(pseudo_id, element);
 }