Prerendered pages are swapped in at browser::Navigate time.

chrome/renderer/prerender/prerender_dispatcher.cc

+#include "chrome/renderer/prerender/prerender_dispatcher.h"
+
+#include "base/logging.h"
+#include "chrome/common/prerender_messages.h"
+#include "googleurl/src/gurl.h"
+
+namespace prerender {
+
+bool PrerenderDispatcher::IsPrerenderURL(const GURL& url) const {
+  return prerender_urls_.find(url) != prerender_urls_.end();
+}
+
+bool PrerenderDispatcher::OnControlMessageReceived(const IPC::Message& message) {
+ bool handled = true;
+  IPC_BEGIN_MESSAGE_MAP(PrerenderDispatcher, message)
+    IPC_MESSAGE_HANDLER(PrerenderMsg_AddPrerenderURL, OnAddPrerenderURL)
+    IPC_MESSAGE_HANDLER(PrerenderMsg_RemovePrerenderURL, OnRemovePrerenderURL)
+    IPC_MESSAGE_UNHANDLED(handled = false)
+  IPC_END_MESSAGE_MAP()
+
+  return handled;
+}
+
+void PrerenderDispatcher::OnAddPrerenderURL(const GURL& url) {
+  PrerenderMap::iterator it = prerender_urls_.find(url);
+  if (it != prerender_urls_.end()) {
+    DCHECK(it->second > 0);
+    it->second += 1;
+  } else {
+    prerender_urls_[url] = 1;
+  }
+}
+
+void PrerenderDispatcher::OnRemovePrerenderURL(const GURL& url) {
+  PrerenderMap::iterator it = prerender_urls_.find(url);
+  // This is possible with a spurious remove.
+  // TODO(cbentzel): We'd also want to send the map of active prerenders when
+  // creating a new render process, so the Add/Remove go relative to that. This may
+  // not be that big of a deal in practice, since the newly created tab is unlikely
+  // to go to the prerendered page.
+  if (it == prerender_urls_.end())
+    return;
+  DCHECK(it->second > 0);
+  if (it->second > 1) {

[dominich, 2012/02/28 16:21:00]

this could also be

it->second -= 1;
if (it->second == 0)
  prerender_urls_.erase(it);

[cbentzel, 2012/02/29 18:16:13]

I like it. Done.

+    it->second -= 1;
+  } else {
+    prerender_urls_.erase(it);
+  }
+}
+
+}  // namespace prerender