[EditingNG] Calculate baseline for empty editable

This patch changes |NGBlockLayoutAlgorithm::Layout()| to calculate baseline
offset for empty line then set it |NGPhysicalBoxFragment| to allow flex layout
algorithm to use valid baseline offset, e.g. "align-items: baseline" CSS
property.

This patch also introduces |LayoutBlock::BaselineForEmptyLine()| to share
baseline offset calculation with legacy latyout.

Note: The test[1][2] verify this change.

[1] third_party/blink/web_tests/css3/flexbox/baseline-for-empty-line.html
[2] third_party/blink/web_tests/inline-block/contenteditable-baseline.html

Bug: 707656
Change-Id: I2a19fb3e457925c03b34ee509b80ba15dff523b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2259675
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781280}

third_party/blink/renderer/core/layout/layout_block.cc

@@ -1708,9 +1708,15 @@ LayoutUnit LayoutBlock::EmptyLineBaseline(
     LineDirectionMode line_direction) const {
   if (!HasLineIfEmpty())
     return LayoutUnit(-1);
+  const auto baseline_offset = BaselineForEmptyLine(line_direction);
+  return baseline_offset ? *baseline_offset : LayoutUnit(-1);
+}
+
+base::Optional<LayoutUnit> LayoutBlock::BaselineForEmptyLine(
+    LineDirectionMode line_direction) const {
   const SimpleFontData* font_data = FirstLineStyle()->GetFont().PrimaryFont();
   if (!font_data)
-    return LayoutUnit(-1);
+    return base::nullopt;
   const auto& font_metrics = font_data->GetFontMetrics();
   const LayoutUnit line_height =
       LineHeight(true, line_direction, kPositionOfInteriorLineBoxes);

third_party/blink/renderer/core/layout/layout_block.h

@@ -402,6 +402,10 @@ class CORE_EXPORT LayoutBlock : public LayoutBox {
   MinMaxSizes PreferredLogicalWidths() const override;
 
   virtual bool HasLineIfEmpty() const;
+  // Returns baseline offset if we can get |SimpleFontData| from primary font.
+  // Or returns no value if we can't get font data.
+  base::Optional<LayoutUnit> BaselineForEmptyLine(
+      LineDirectionMode line_direction) const;
 
  protected:
   virtual void AdjustInlineDirectionLineBounds(
@@ -477,6 +481,9 @@ class CORE_EXPORT LayoutBlock : public LayoutBox {
            hit_test_action == kHitTestChildBlockBackground;
   }
 
+  // Returns baseline offset of this block if is empty editable or having
+  // CSS property "--internal-empty-line-height"fabricated", otherwise
+  // returns |LayoutUnit(-1)|.
   LayoutUnit EmptyLineBaseline(LineDirectionMode line_direction) const;
 
  private:

third_party/blink/renderer/core/layout/ng/ng_block_layout_algorithm.cc

@@ -732,6 +732,15 @@ inline scoped_refptr<const NGLayoutResult> NGBlockLayoutAlgorithm::Layout(
     intrinsic_block_size_ +=
         std::max(intrinsic_block_size_,
                  Node().GetLayoutBox()->LogicalHeightForEmptyLine());
+    // Test [1][2] require baseline offset for empty editable.
+    // [1] css3/flexbox/baseline-for-empty-line.html
+    // [2] inline-block/contenteditable-baseline.html
+    const LayoutBlock* const layout_block =
+        To<LayoutBlock>(Node().GetLayoutBox());
+    if (auto baseline_offset = layout_block->BaselineForEmptyLine(
+            layout_block->IsHorizontalWritingMode() ? kHorizontalLine
+                                                    : kVerticalLine))
+      container_builder_.SetBaseline(*baseline_offset);
   }
 
   // Collapse annotation overflow and padding.