Introduce TextIteratorTextState::GetTextForTesting() as replacement of Substring()

This patch introduces |TextIteratorTextState::GetTextForTesting()| as
replacement of |Substring()|, because it is called only one place and it takes
whole text, for improving code health.

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: Id71f35c82c1906dfea9503a77506a6a437246bae
Reviewed-on: https://chromium-review.googlesource.com/1125584Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572519}

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

@@ -123,8 +123,7 @@ std::string TextIteratorTest::IterateWithIterator(
   String text_chunks;
   for (; !iterator.AtEnd(); iterator.Advance()) {
     text_chunks.append('[');
-    text_chunks.append(
-        iterator.GetText().Substring(0, iterator.GetText().length()));
+    text_chunks.append(iterator.GetText().GetTextForTesting());
     text_chunks.append(']');
   }
   return std::string(text_chunks.Utf8().data());

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

@@ -73,18 +73,10 @@ UChar TextIteratorTextState::CharacterAt(unsigned index) const {
   return text_[text_start_offset_ + index];
 }
 
-String TextIteratorTextState::Substring(unsigned position,
-                                        unsigned length) const {
-  SECURITY_DCHECK(position <= this->length());
-  SECURITY_DCHECK(position + length <= this->length());
-  if (!length)
-    return g_empty_string;
-  if (single_character_buffer_) {
-    DCHECK_EQ(position, 0u);
-    DCHECK_EQ(length, 1u);
+String TextIteratorTextState::GetTextForTesting() const {
+  if (single_character_buffer_)
     return String(&single_character_buffer_, 1);
-  }
-  return text_.Substring(text_start_offset_ + position, length);
+  return text_.Substring(text_start_offset_, length());
 }
 
 void TextIteratorTextState::AppendTextToStringBuilder(

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

@@ -49,7 +49,7 @@ class CORE_EXPORT TextIteratorTextState {
   // Return properties of the current text.
   unsigned length() const { return text_length_; }
   UChar CharacterAt(unsigned index) const;
-  String Substring(unsigned position, unsigned length) const;
+  String GetTextForTesting() const;
   void AppendTextToStringBuilder(StringBuilder&,
                                  unsigned position = 0,
                                  unsigned max_length = UINT_MAX) const;