Move ClosePage() from RenderViewHost to WebContents.

ClosePage is a page-level concept and because of the deprecation of
RenderViewHost, it has to move. WebContents is the object representing
page, so I'm moving this public API to it.

BUG=304341, 478281

Review URL: https://codereview.chromium.org/1094153002

Cr-Commit-Position: refs/heads/master@{#327719}
diff --git a/content/browser/web_contents/web_contents_impl.h b/content/browser/web_contents/web_contents_impl.h
index 350d1ca0..8ad8a8ba 100644
--- a/content/browser/web_contents/web_contents_impl.h
+++ b/content/browser/web_contents/web_contents_impl.h
@@ -235,6 +235,7 @@
   RenderViewHostImpl* GetRenderViewHost() const override;
   int GetRoutingID() const override;
   RenderWidgetHostView* GetRenderWidgetHostView() const override;
+  void ClosePage() override;
   RenderWidgetHostView* GetFullscreenRenderWidgetHostView() const override;
   SkColor GetThemeColor() const override;
   WebUI* CreateWebUI(const GURL& url) override;