[cleanup] Refactor FlexLine::ComputeLineItemsPosition.

In preparation for:
https://chromium-review.googlesource.com/c/chromium/src/+/2490323

There should be no behaviour change - just moving things around to
reduce deltas of subsequent behaviour changing patches.

Change-Id: Iac5714cb492cfaf7b2dc5117cb39d4f1b7b9b452
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2507336
Commit-Queue: David Grogan <dgrogan@chromium.org>
Reviewed-by: David Grogan <dgrogan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822550}

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

@@ -482,6 +482,9 @@ LayoutUnit FlexLine::ApplyMainAxisAutoMarginAdjustment() {
 void FlexLine::ComputeLineItemsPosition(LayoutUnit main_axis_start_offset,
                                         LayoutUnit main_axis_end_offset,
                                         LayoutUnit& cross_axis_offset) {
+  const auto& style = algorithm_->StyleRef();
+  const bool is_webkit_box = style.IsDeprecatedWebkitBox();
+
   main_axis_offset_ = main_axis_start_offset;
   // Recalculate the remaining free space. The adjustment for flex factors
   // between 0..1 means we can't just use remainingFreeSpace here.
@@ -493,43 +496,41 @@ void FlexLine::ComputeLineItemsPosition(LayoutUnit main_axis_start_offset,
       (line_items_.size() - 1) * algorithm_->gap_between_items_;
 
   const StyleContentAlignmentData justify_content =
-      FlexLayoutAlgorithm::ResolvedJustifyContent(*algorithm_->Style());
+      FlexLayoutAlgorithm::ResolvedJustifyContent(style);
 
-  LayoutUnit auto_margin_offset = ApplyMainAxisAutoMarginAdjustment();
+  const LayoutUnit auto_margin_offset = ApplyMainAxisAutoMarginAdjustment();
   const LayoutUnit available_free_space = remaining_free_space_;
-  LayoutUnit initial_position =
+  const LayoutUnit initial_position =
       FlexLayoutAlgorithm::InitialContentPositionOffset(
-          algorithm_->StyleRef(), available_free_space, justify_content,
-          line_items_.size());
-
-  LayoutUnit main_axis_offset = initial_position;
+          style, available_free_space, justify_content, line_items_.size());
   sum_justify_adjustments_ += initial_position;
-  LayoutUnit max_descent;  // Used when align-items: baseline.
-  LayoutUnit max_child_cross_axis_extent;
+  LayoutUnit main_axis_offset = initial_position + main_axis_start_offset;
+
+  LayoutUnit logical_width_when_flipped = container_logical_width_;
+  LayoutUnit flipped_offset;
   bool should_flip_main_axis;
   if (algorithm_->IsNGFlexBox()) {
-    should_flip_main_axis =
-        algorithm_->StyleRef().ResolvedIsRowReverseFlexDirection();
+    should_flip_main_axis = style.ResolvedIsRowReverseFlexDirection();
   } else {
-    should_flip_main_axis =
-        !algorithm_->StyleRef().ResolvedIsColumnFlexDirection() &&
+    should_flip_main_axis = !style.ResolvedIsColumnFlexDirection() &&
                             !algorithm_->IsLeftToRightFlow();
-  }
-  LayoutUnit width_when_flipped = container_logical_width_;
-  LayoutUnit flipped_offset;
-  // -webkit-box, always did layout starting at 0. ltr and reverse were handled
-  // by reversing the order of iteration. OTOH, flex always iterates in order
-  // and flips the main coordinate. The following gives the same behavior for
-  // -webkit-box while using the same iteration order as flex does by changing
-  // how the flipped coordinate is calculated.
-  if (should_flip_main_axis && algorithm_->StyleRef().IsDeprecatedWebkitBox()) {
+
+    // -webkit-box, always did layout starting at 0. ltr and reverse were
+    // handled by reversing the order of iteration. OTOH, flex always iterates
+    // in order and flips the main coordinate. The following gives the same
+    // behavior for -webkit-box while using the same iteration order as flex
+    // does by changing how the flipped coordinate is calculated.
+    if (should_flip_main_axis && is_webkit_box) {
       // -webkit-box only distributed space when > 0.
-    width_when_flipped =
+      logical_width_when_flipped =
           total_item_size + available_free_space.ClampNegativeToZero();
       flipped_offset = main_axis_end_offset;
-  } else {
-    main_axis_offset += main_axis_start_offset;
+      main_axis_offset -= main_axis_start_offset;
+    }
   }
+
+  LayoutUnit max_descent;  // Used when align-items: baseline.
+  LayoutUnit max_child_cross_axis_extent;
   for (size_t i = 0; i < line_items_.size(); ++i) {
     FlexItem& flex_item = line_items_[i];
 
@@ -561,7 +562,7 @@ void FlexLine::ComputeLineItemsPosition(LayoutUnit main_axis_start_offset,
     // on the left. This will be fixed later in
     // LayoutFlexibleBox::FlipForRightToLeftColumn.
     flex_item.desired_location_ = LayoutPoint(
-        should_flip_main_axis ? width_when_flipped - main_axis_offset -
+        should_flip_main_axis ? logical_width_when_flipped - main_axis_offset -
                                     child_main_extent + flipped_offset
                               : main_axis_offset,
         cross_axis_offset + flex_item.FlowAwareMarginBefore());