[FragmentItem] Fix reusing lines of `::first-line`

When `::first-line` state was changed, some lines may be
still reusable, but reusing may put the fragment tree in a
strange state where the block container doesn't think
different text/styles are needed for `::first-line`, but
|NGFragmentItem| may require the style/text for
`::first-line`.

This patch disallows reusing lines from `::first-line` if
new block container does not have different `::first-line`
text/style.

Bug: 1104533
Change-Id: Ieb8745726ea8664480c248bca2ec991813027f11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2291181
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787610}

third_party/blink/renderer/core/layout/ng/inline/ng_fragment_items.h

@@ -36,6 +36,9 @@ class CORE_EXPORT NGFragmentItems {
     return items_[0];
   }
 
+  // Text content for `::first-line`. Available only if `::first-line` has
+  // different style than non-first-line style.
+  const String& FirstLineText() const { return first_line_text_content_; }
   const String& Text(bool first_line) const {
     return UNLIKELY(first_line) ? first_line_text_content_ : text_content_;
   }

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

@@ -156,6 +156,13 @@ NGFragmentItemsBuilder::AddPreviousItems(
     DCHECK(node_);
     DCHECK(container_builder);
     DCHECK(text_content_);
+
+    if (UNLIKELY(items.FirstLineText() && !first_line_text_content_)) {
+      // Don't reuse previous items if they have different `::first-line` style
+      // but |this| doesn't. Reaching here means that computed style doesn't
+      // change, but |NGFragmentItem| has wrong |NGStyleVariant|.
+      return AddPreviousItemsResult();
+    }
   } else {
     DCHECK(!container_builder);
     DCHECK(!text_content_);