Guard against duplicate adds in PrerenderLinkManager.

chrome/browser/prerender/prerender_link_manager.cc

Index: chrome/browser/prerender/prerender_link_manager.cc
diff --git a/chrome/browser/prerender/prerender_link_manager.cc b/chrome/browser/prerender/prerender_link_manager.cc
index 8b099b9a8a9e663d14f4d4c65b0a0dac2837984a..d27a81e9264d230db4a31b1ea9a4d37685b70009 100644
--- a/chrome/browser/prerender/prerender_link_manager.cc
+++ b/chrome/browser/prerender/prerender_link_manager.cc
@@ -67,9 +67,11 @@ void PrerenderLinkManager::OnAddPrerender(int launcher_child_id,
                                           const content::Referrer& referrer,
                                           const gfx::Size& size,
                                           int render_view_route_id) {
-  DCHECK_EQ(static_cast<LinkPrerender*>(NULL),
-            FindByLauncherChildIdAndPrerenderId(launcher_child_id,
-                                                prerender_id));
+  // TODO(gavinp): Determine why WebKit appears to be sending duplicate adds,
+  // and prevent it.
+  if (FindByLauncherChildIdAndPrerenderId(launcher_child_id, prerender_id))
+    return;
+
   content::RenderProcessHost* rph =
       content::RenderProcessHost::FromID(launcher_child_id);
   // Guests inside <webview> do not support cross-process navigation and so we