[layoutng] margin: auto support

third_party/WebKit/Source/core/layout/ng/ng_length_utils.cc

Index: third_party/WebKit/Source/core/layout/ng/ng_length_utils.cc
diff --git a/third_party/WebKit/Source/core/layout/ng/ng_length_utils.cc b/third_party/WebKit/Source/core/layout/ng/ng_length_utils.cc
index 87894828d679bff724a9b0adefe525186176be38..54d4172ea63a45e1d55d251fb2db70f6d9dcf977 100644
--- a/third_party/WebKit/Source/core/layout/ng/ng_length_utils.cc
+++ b/third_party/WebKit/Source/core/layout/ng/ng_length_utils.cc
@@ -5,6 +5,7 @@
 #include "core/layout/ng/ng_length_utils.h"
 
 #include "core/layout/ng/ng_constraint_space.h"
+#include "core/layout/ng/ng_fragment.h"
 #include "core/style/ComputedStyle.h"
 #include "platform/LayoutUnit.h"
 #include "platform/Length.h"
@@ -283,4 +284,23 @@ NGBoxStrut computePadding(const NGConstraintSpace& constraintSpace,
   return padding;
 }
 
+void ApplyAutoMargins(const NGConstraintSpace& constraint_space,
+                      const ComputedStyle& style,
+                      const NGFragment& fragment,
+                      NGBoxStrut& margins) {
+  const LayoutUnit used_space = fragment.InlineSize() + margins.InlineSum();
+  const LayoutUnit available_space =
+      constraint_space.ContainerSize().inline_size - used_space;
+  if (available_space < LayoutUnit())
+    return;
+  if (style.marginStart().isAuto() && style.marginEnd().isAuto()) {
+    margins.inline_start = available_space / 2;
+    margins.inline_end = available_space - margins.inline_start;
+  } else if (style.marginStart().isAuto()) {
+    margins.inline_start = available_space;
+  } else if (style.marginEnd().isAuto()) {
+    margins.inline_end = available_space;
+  }
+}
+
 }  // namespace blink