List marker_container should be a self-collapsing block.

Marker_container is a zero height anonymous block and marker is its
only child. Marker_container positioned at the side of list content,
it shouldn't break margin collapse.

Gecko already do this.

Change-Id: I8a47605e80eb927165c359a4bfcd66da0f847e4d
Reviewed-on: https://chromium-review.googlesource.com/773766
Commit-Queue: cathie chen <cathiechen@tencent.com>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521290}

third_party/WebKit/LayoutTests/fast/lists/list-and-margin-collapse.html

+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<ul id="target" style="margin-top:100px;">
+  <li>
+    <div style="overflow:hidden; margin-top:100px; height:25px;"><a href="#">xxx</a></div>
+  </li>
+</ul>
+<div id="log"></div>
+<script>
+test(function() {
+  var height = document.getElementById("target").clientHeight;
+  assert_equals(height, 25, "the height of ul should be 25px")
+}, "list and margin collapse");
+</script>

third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp

@@ -387,6 +387,16 @@ bool LayoutBlockFlow::CheckIfIsSelfCollapsingBlock() const {
   if (has_auto_height || ((logical_height_length.IsFixed() ||
                            logical_height_length.IsPercentOrCalc()) &&
                           logical_height_length.IsZero())) {
+    // Marker_container should be a self-collapsing block. Marker_container is a
+    // zero height anonymous block and marker is its only child.
+    if (logical_height_length.IsFixed() && logical_height_length.IsZero() &&
+        IsAnonymous() && Parent() && Parent()->IsListItem()) {
+      LayoutObject* first_child = FirstChild();
+      if (first_child && first_child->IsListMarker() &&
+          !first_child->NextSibling())
+        return true;
+    }
+
     // If the block has inline children, see if we generated any line boxes.
     // If we have any line boxes, then we can't be self-collapsing, since we
     // have content.