DOM: Add method doc on Element::GetShadowRoot

This is trivial but it's unclear to someone unfamiliar with the code
whether this is going to get the attached root or the containing root
(and that method is 2 layers of inheritance away in Node.h).

Change-Id: I804664d12e2a245db5eff5b5dc7b77a15b995dc0
Reviewed-on: https://chromium-review.googlesource.com/1009302Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550082}

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

@@ -497,6 +497,7 @@ class CORE_EXPORT Element : public ContainerNode {
   ShadowRoot& AttachShadowRootInternal(ShadowRootType,
                                        bool delegates_focus = false);
 
+  // Returns the shadow root attached to this element if it is a shadow host.
   ShadowRoot* GetShadowRoot() const;
   ShadowRoot* OpenShadowRoot() const;
   ShadowRoot* ClosedShadowRoot() const;