[LayoutNG] Refactor the usage of GetCharacterRange in NGCaretRect

This patch refactors inefficient usage of GetCharacterRange
as discussed in CL:979732.

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I1d1b135eaf8d7a12f0fd918316428d5bf30cbcc0
Reviewed-on: https://chromium-review.googlesource.com/981848Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547079}

third_party/WebKit/Source/core/layout/ng/inline/ng_caret_rect.cc

@@ -264,17 +264,9 @@ NGPhysicalOffsetRect ComputeLocalCaretRectAtTextOffset(
       is_horizontal ? fragment.Size().height : fragment.Size().width;
   LayoutUnit caret_top;
 
-  LayoutUnit caret_left;
-  if (!fragment.IsLineBreak()) {
-    unsigned offset_in_fragment = offset - fragment.StartOffset();
-    const ShapeResult* shape_result = fragment.TextShapeResult();
-    DCHECK(shape_result);
-    CharacterRange character_range =
-        shape_result->GetCharacterRange(offset_in_fragment, offset_in_fragment);
-
-    LayoutUnit caret_center = LayoutUnit(character_range.start);
-    caret_left = caret_center - caret_width / 2;
-  }
+  LayoutUnit caret_left = fragment.InlinePositionForOffset(offset);
+  if (!fragment.IsLineBreak())
+    caret_left -= caret_width / 2;
 
   if (!is_horizontal) {
     std::swap(caret_top, caret_left);

third_party/WebKit/Source/core/layout/ng/inline/ng_physical_text_fragment.cc

@@ -15,6 +15,24 @@
 
 namespace blink {
 
+LayoutUnit NGPhysicalTextFragment::InlinePositionForOffset(
+    unsigned offset) const {
+  DCHECK_GE(offset, start_offset_);
+  DCHECK_LE(offset, end_offset_);
+
+  offset -= start_offset_;
+  if (shape_result_) {
+    return LayoutUnit::FromFloatRound(shape_result_->PositionForOffset(offset));
+  }
+
+  // All non-flow controls have ShapeResult.
+  DCHECK(IsFlowControl());
+  DCHECK_EQ(1u, Length());
+  if (!offset || UNLIKELY(IsRtl(Style().Direction())))
+    return LayoutUnit();
+  return IsHorizontal() ? Size().width : Size().height;
+}
+
 NGPhysicalOffsetRect NGPhysicalTextFragment::LocalRect(
     unsigned start_offset,
     unsigned end_offset) const {

third_party/WebKit/Source/core/layout/ng/inline/ng_physical_text_fragment.h

@@ -105,6 +105,10 @@ class CORE_EXPORT NGPhysicalTextFragment final : public NGPhysicalFragment {
     return IsHorizontal() ? kAlphabeticBaseline : kIdeographicBaseline;
   }
 
+  // Compute the inline position from text offset, in logical coordinate
+  // relative to this fragment.
+  LayoutUnit InlinePositionForOffset(unsigned offset) const;
+
   // The layout box of text in (start, end) range in local coordinate.
   // Start and end offsets must be between StartOffset() and EndOffset().
   NGPhysicalOffsetRect LocalRect(unsigned start_offset,