Remove Document::referrer_policy_

It's not referenced at all.

Bug: None
Change-Id: I28ea78f8ab424564aae794690dfae737b9a7b054
Reviewed-on: https://chromium-review.googlesource.com/519202Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#475871}

third_party/WebKit/Source/core/dom/Document.h

@@ -1631,8 +1631,6 @@ class CORE_EXPORT Document : public ContainerNode,
   ViewportDescription legacy_viewport_description_;
   Length viewport_default_min_width_;
 
-  ReferrerPolicy referrer_policy_;
-
   DocumentTiming document_timing_;
   Member<MediaQueryMatcher> media_query_matcher_;
   bool write_recursion_is_too_deep_;