Do not create proxy hosts in the subtree of navigating frame.

content/browser/frame_host/render_frame_host_manager.cc

Index: content/browser/frame_host/render_frame_host_manager.cc
diff --git a/content/browser/frame_host/render_frame_host_manager.cc b/content/browser/frame_host/render_frame_host_manager.cc
index 6fcb94122deb087aa949dec2e8299f08e17b58d1..1183272abe63d60c63260ef5c49e37a72e8b2a21 100644
--- a/content/browser/frame_host/render_frame_host_manager.cc
+++ b/content/browser/frame_host/render_frame_host_manager.cc
@@ -1401,8 +1401,22 @@ void RenderFrameHostManager::CommitPending() {
           linked_ptr<RenderFrameHostImpl>(old_render_frame_host.release());
     }
   } else {
-    CHECK(proxy_hosts_.find(render_frame_host_->GetSiteInstance()->GetId()) ==
-          proxy_hosts_.end());
+    // In --site-per-process, delete the proxy for the SiteInstance we are
+    // committing. In the case of main frame, the proxy wraps the
+    // |old_render_frame_host|, so it must be left intact.
+    // TODO(nasko): When committing into an existing process, the proxy
+    // should be swapped in the frame tree with the real frame, but this is
+    // not yet the case. Revisit this to ensure the necessary context is
+    // preserved.
+    if (CommandLine::ForCurrentProcess()->HasSwitch(
+            switches::kSitePerProcess) && !is_main_frame) {
+      RenderFrameProxyHostMap::iterator iter =
+          proxy_hosts_.find(render_frame_host_->GetSiteInstance()->GetId());
+      if (iter != proxy_hosts_.end()) {
+        delete iter->second;
+        proxy_hosts_.erase(iter);
+      }
+    }
 
     // Capture the active view count on the old RFH SiteInstance, since the
     // ownership might be passed into the proxy and the pointer will be
@@ -1437,6 +1451,11 @@ void RenderFrameHostManager::CommitPending() {
       }
     }
   }
+
+  // After all is done, there must never be a proxy in the list which has the
+  // same SiteInstance as the current RenderFrameHost.
+  CHECK(proxy_hosts_.find(render_frame_host_->GetSiteInstance()->GetId()) ==
+        proxy_hosts_.end());
 }
 
 void RenderFrameHostManager::ShutdownRenderFrameProxyHostsInSiteInstance(