Check for already existing entry when adding to SiteProcessMap.

content/browser/renderer_host/render_process_host_impl.cc

Index: content/browser/renderer_host/render_process_host_impl.cc
diff --git a/content/browser/renderer_host/render_process_host_impl.cc b/content/browser/renderer_host/render_process_host_impl.cc
index a9c2314efe81b1ec63a27eeed6b3a28f3a458843..67298e50e9cc42b8202596de8d955afc2b42bfd7 100644
--- a/content/browser/renderer_host/render_process_host_impl.cc
+++ b/content/browser/renderer_host/render_process_host_impl.cc
@@ -335,7 +335,13 @@ class SiteProcessMap : public base::SupportsUserData::Data {
   SiteProcessMap() {}
 
   void RegisterProcess(const std::string& site, RenderProcessHost* process) {
-    map_[site] = process;
+    // There could already exist a site to process mapping due to races between
+    // two WebContents with blank SiteInstances. If that occurs, keeping the
+    // exising entry and not overwriting it is a predictable behavior that is
+    // safe.
+    SiteToProcessMap::iterator i = map_.find(site);
+    if (i == map_.end())
+      map_[site] = process;
   }
 
   RenderProcessHost* FindProcess(const std::string& site) {