Resolve inline margins in over-constrained situations correctly.

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

Index: third_party/WebKit/Source/core/layout/LayoutBoxTest.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBoxTest.cpp b/third_party/WebKit/Source/core/layout/LayoutBoxTest.cpp
index 0b6c93be89dd7563d402c3c1698fb33ec0dceb5c..8b12dac48298253ad8a14501fbf83b625ec72247 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxTest.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxTest.cpp
@@ -263,4 +263,18 @@ TEST_F(LayoutBoxTest, LocalVisualRectWithMaskAndOverflowClip) {
   EXPECT_EQ(LayoutRect(0, 0, 100, 100), target->LocalVisualRect());
 }
 
+TEST_F(LayoutBoxTest, ChildWidthBiggerThanParentAutoMarign) {

[mstensho (USE GERRIT), 2017/04/27 12:37:28]

*Margin*

But I really wonder if you could write a LayoutTest instead.
LayoutTests/fast/block/ is probably a suitable home for it. This isn't really a
unit test, and besides, if you write a LayoutTest, it will also be testable in
the upcoming layout engine (NG).

It should be possible to use getComputedStyle(target).marginRight

I see that this is your first commit ever (Welcome!), so if you wonder how we
prefer layout tests to be written these days, you can for example take a look at
LayoutTests/fast/block/scrollbar-wider-than-border-box.html

Bonus point if you also write an additional test for direction:rtl and
margin-left. :)

+  SetBodyInnerHTML(
+      "<div style='"
+      "  width: 400px; border: 3px solid black;"
+      "'>"
+      "  <div id='target' style='"
+      "    margin-left: 10px; width: 500px; border: 3px solid gray; "
+      "margin-right: auto;"
+      "  '></div>"
+      "</div>");
+  LayoutBox* target = ToLayoutBox(GetLayoutObjectByElementId("target"));
+  EXPECT_EQ(LayoutUnit(0), target->MarginEnd());
+}
+
 }  // namespace blink