[layoutng] Correctly set the exclusion space when cloning

Due to variable shadowing the exclusion space was always null after
cloning, causing problems with fragment caching.

R=eae@chromium.org
BUG=635619

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I5aca88fdc693f1ad64848f43a108953bf1fc8f65
Reviewed-on: https://chromium-review.googlesource.com/986992
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547045}

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

@@ -60,8 +60,7 @@ scoped_refptr<NGLayoutResult> NGLayoutResult::CloneWithoutOffset() const {
   // TODO(layoutng) Replace this with DCHECK(exclusion_space_) when
   // callers guarantee exclusion_space_ != null.
   if (exclusion_space_) {
-    std::unique_ptr<const NGExclusionSpace> exclusion_space(
-        std::make_unique<NGExclusionSpace>(*exclusion_space_));
+    exclusion_space = std::make_unique<NGExclusionSpace>(*exclusion_space_);
   }
   return base::AdoptRef(new NGLayoutResult(
       physical_fragment_->CloneWithoutOffset(), oof_positioned_descendants,