Remove TODO comment

Element::GetShadowRoot() doesn't have to be inlined.
That would cause element.h to depend on element_rare_data.h, which we wouldn't
want.

Change-Id: Ib20c6c5b2e3b13510c7eb142704df53439bfcc9d
Reviewed-on: https://chromium-review.googlesource.com/1248344
Commit-Queue: Hayato Ito <hayato@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594607}

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

@@ -2728,7 +2728,6 @@ ShadowRoot& Element::CreateAndAttachShadowRoot(ShadowRootType type) {
   return *shadow_root;
 }
 
-// TODO(kochi): inline this.
 ShadowRoot* Element::GetShadowRoot() const {
   return HasRareData() ? GetElementRareData()->GetShadowRoot() : nullptr;
 }