Make CharacterIterator::CurrentContainer() to return const Node&

This patch changes |CharacterIterator::CurrentContainer()| to return
|const Node&| instead of |const Node*| since it always non-null for improving
code health.

This patch is a follow-up of the patch[1].

[1] http://crrev.com/c/1050053 Make TextIterator::CurrentContainer() to return
const Node&.

Change-Id: If2dc187244e4e8a489b56f9855b786c191579d9b
Reviewed-on: https://chromium-review.googlesource.com/1051138
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557151}

third_party/blink/renderer/core/editing/iterators/character_iterator.cc

@@ -63,8 +63,8 @@ const Document& CharacterIteratorAlgorithm<Strategy>::OwnerDocument() const {
 }
 
 template <typename Strategy>
-const Node* CharacterIteratorAlgorithm<Strategy>::CurrentContainer() const {
-  return &text_iterator_.CurrentContainer();
+const Node& CharacterIteratorAlgorithm<Strategy>::CurrentContainer() const {
+  return text_iterator_.CurrentContainer();
 }
 
 template <typename Strategy>

third_party/blink/renderer/core/editing/iterators/character_iterator.h

@@ -65,7 +65,7 @@ class CORE_EXPORT CharacterIteratorAlgorithm {
   int CharacterOffset() const { return offset_; }
 
   const Document& OwnerDocument() const;
-  const Node* CurrentContainer() const;
+  const Node& CurrentContainer() const;
   int StartOffset() const;
   int EndOffset() const;