Refactor SelectionModifier's internal function DirectionOf()

This patch refactors the function by extracing DirectionAt() out of it,
so that the logic of getting direction from VisiblePosition is
abstracted. It also reduces one client of InlineBox, making it easier to
transition to LayoutNG.

Bug: 877263
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: Ifba556e3fb03b7bfa6e4ed27df5e39d07a45eb7c
Reviewed-on: https://chromium-review.googlesource.com/1187630Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585701}

third_party/blink/renderer/core/editing/selection_modifier.cc

@@ -137,23 +137,30 @@ TextDirection SelectionModifier::DirectionOfEnclosingBlock() const {
              : TextDirection::kLtr;
 }
 
-static TextDirection DirectionOf(const VisibleSelection& visible_selection) {
-  const InlineBox* start_box = nullptr;
-  const InlineBox* end_box = nullptr;
-  // Cache the VisiblePositions because visibleStart() and visibleEnd()
-  // can cause layout, which has the potential to invalidate lineboxes.
-  const VisiblePosition& start_position = visible_selection.VisibleStart();
-  const VisiblePosition& end_position = visible_selection.VisibleEnd();
-  if (start_position.IsNotNull())
-    start_box = ComputeInlineBoxPosition(start_position).inline_box;
-  if (end_position.IsNotNull())
-    end_box = ComputeInlineBoxPosition(end_position).inline_box;
-  if (start_box && end_box && start_box->Direction() == end_box->Direction())
-    return start_box->Direction();
+namespace {
+
+base::Optional<TextDirection> DirectionAt(const VisiblePosition& position) {
+  if (position.IsNull())
+    return base::nullopt;
+  if (const InlineBox* box = ComputeInlineBoxPosition(position).inline_box)
+    return box->Direction();
+  return base::nullopt;
+}
+
+TextDirection DirectionOf(const VisibleSelection& visible_selection) {
+  base::Optional<TextDirection> maybe_start_direction =
+      DirectionAt(visible_selection.VisibleStart());
+  base::Optional<TextDirection> maybe_end_direction =
+      DirectionAt(visible_selection.VisibleEnd());
+  if (maybe_start_direction.has_value() && maybe_end_direction.has_value() &&
+      maybe_start_direction.value() == maybe_end_direction.value())
+    return maybe_start_direction.value();
 
   return DirectionOfEnclosingBlockOf(visible_selection.Extent());
 }
 
+}  // namespace
+
 TextDirection SelectionModifier::DirectionOfSelection() const {
   return DirectionOf(selection_);
 }