[LayoutNG] Add DCHECK to NGLineBreaker::CreateBreakToken

The assumption is |NGLineBreaker::CreateBreakToken| always
has a valid style. This patch adds DCHECK for the assumption.

TBR=mstensho@chromium.org

Bug: 1013040
Change-Id: Idbe5594136c8b1b9c4556dacf81631af49a56a06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1855066Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705012}

third_party/blink/renderer/core/layout/ng/inline/ng_line_breaker.cc

@@ -1898,7 +1898,9 @@ void NGLineBreaker::MoveToNextOf(const NGInlineItemResult& item_result) {
 
 scoped_refptr<NGInlineBreakToken> NGLineBreaker::CreateBreakToken(
     const NGLineInfo& line_info) const {
+  DCHECK(current_style_);
   const Vector<NGInlineItem>& items = Items();
+  DCHECK_LE(item_index_, items.size());
   if (item_index_ >= items.size())
     return NGInlineBreakToken::Create(node_);
   return NGInlineBreakToken::Create(