[FlexNG] Prefer a baseline from an empty child with baseline alignment.

Previously if a fragment didn't provide a baseline, and had baseline
alignment we wouldn't consider it as having the default baseline.

This was incorrect. Causing fast/css-grid-layout/grid-baseline.html
to fail when FlexNG was turned on:
https://test-results.appspot.com/data/layout_results/linux-rel/294412/webkit_layout_tests%20%28with%20patch%29/layout-test-results/results.html

Instead this patch changes the flex baseline propagation logic to prefer
any flex-item with baseline alignment, even if we have to synthesize
the baseline.

Bug: 845235
Change-Id: I4cc40da42a8d6be41b028f1170246b8004f0654d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2024546Reviewed-by: David Grogan <dgrogan@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736082}

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

@@ -856,26 +856,21 @@ void NGFlexLayoutAlgorithm::PropagateBaselineFromChild(
   NGBoxFragment fragment(ConstraintSpace().GetWritingMode(),
                          ConstraintSpace().Direction(), physical_fragment);
 
-  if (base::Optional<LayoutUnit> baseline = fragment.Baseline()) {
-    LayoutUnit baseline_offset = block_offset + *baseline;
-
-    // We prefer a baseline from a child with baseline alignment, and no
-    // auto-margins in the cross axis.
-    if (FlexLayoutAlgorithm::AlignmentForChild(Style(), flex_item.style) ==
-            ItemPosition::kBaseline &&
-        !flex_item.HasAutoMarginsInCrossAxis()) {
-      container_builder_.SetBaseline(baseline_offset);
-      return;
-    }
-
-    // Set the fallback baseline to this (if not set yet).
-    *fallback_baseline = fallback_baseline->value_or(baseline_offset);
+  LayoutUnit baseline_offset =
+      block_offset + fragment.Baseline().value_or(fragment.BlockSize());
+
+  // We prefer a baseline from a child with baseline alignment, and no
+  // auto-margins in the cross axis (even if we have to synthesize the
+  // baseline).
+  if (FlexLayoutAlgorithm::AlignmentForChild(Style(), flex_item.style) ==
+          ItemPosition::kBaseline &&
+      !flex_item.HasAutoMarginsInCrossAxis()) {
+    container_builder_.SetBaseline(baseline_offset);
     return;
   }
 
-  // Use the block-end border-box edge as the last resort baseline.
-  *fallback_baseline =
-      fallback_baseline->value_or(block_offset + fragment.BlockSize());
+  // Set the fallback baseline if it doesn't have a value yet.
+  *fallback_baseline = fallback_baseline->value_or(baseline_offset);
 }
 
 base::Optional<MinMaxSize> NGFlexLayoutAlgorithm::ComputeMinMaxSize(