Fix incorrectly calculated content_size for fragments that create new FC

third_party/WebKit/Source/core/layout/ng/ng_fragment_builder.h

Index: third_party/WebKit/Source/core/layout/ng/ng_fragment_builder.h
diff --git a/third_party/WebKit/Source/core/layout/ng/ng_fragment_builder.h b/third_party/WebKit/Source/core/layout/ng/ng_fragment_builder.h
index 524ac679dc2c58bfc2afd789fd7d5f2499738362..44e85bc397b95e8fc4595bfcf9b8e19e322dbea0 100644
--- a/third_party/WebKit/Source/core/layout/ng/ng_fragment_builder.h
+++ b/third_party/WebKit/Source/core/layout/ng/ng_fragment_builder.h
@@ -30,6 +30,12 @@ class CORE_EXPORT NGFragmentBuilder final
   NGFragmentBuilder& SetMarginStrutBlockStart(const NGMarginStrut& from);
   NGFragmentBuilder& SetMarginStrutBlockEnd(const NGMarginStrut& from);
 
+  // Clears the fragment's margin strut.
+  // This is needed if we know that the fragment's margin strut will not
+  // participate in Collapsing Margins, for example if the fragment establishes
+  // a new formatting context.
+  NGFragmentBuilder& ClearMarginStrut();
+
   // Offsets are not supposed to be set during fragment construction, so we
   // do not provide a setter here.