Check for already existing entry when adding to SiteProcessMap.

content/browser/frame_host/navigator_impl.cc

Index: content/browser/frame_host/navigator_impl.cc
diff --git a/content/browser/frame_host/navigator_impl.cc b/content/browser/frame_host/navigator_impl.cc
index 07f3cc9d1eb6137155132c4f146032ea6b1efe7d..a10793e4a2ca4295fb26f2fd507bf061d2902a1a 100644
--- a/content/browser/frame_host/navigator_impl.cc
+++ b/content/browser/frame_host/navigator_impl.cc
@@ -625,12 +625,12 @@ void NavigatorImpl::DidNavigate(
   }
 
   // Update the site of the SiteInstance if it doesn't have one yet, unless
-  // assigning a site is not necessary for this URL.  In that case, the
-  // SiteInstance can still be considered unused until a navigation to a real
-  // page.
+  // assigning a site is not necessary for this URL or the commit was for an
+  // error page.  In that case, the SiteInstance can still be considered unused
+  // until a navigation to a real page.
   SiteInstanceImpl* site_instance = render_frame_host->GetSiteInstance();
-  if (!site_instance->HasSite() &&
-      ShouldAssignSiteForURL(params.url)) {
+  if (!site_instance->HasSite() && ShouldAssignSiteForURL(params.url) &&
+      !params.url_is_unreachable) {
     site_instance->SetSite(params.url);
   }