Enable spare RenderProcessHost to be preinitialized.

content/browser/browser_context.cc

Index: content/browser/browser_context.cc
diff --git a/content/browser/browser_context.cc b/content/browser/browser_context.cc
index cb740d98b359cf39d2356333b28b93686623b7a3..9b948b39b9decc449a466ef2b2648e7a9c44072d 100644
--- a/content/browser/browser_context.cc
+++ b/content/browser/browser_context.cc
@@ -368,8 +368,10 @@ void BrowserContext::NotifyWillBeDestroyed(BrowserContext* browser_context) {
            RenderProcessHost::AllHostsIterator();
        !host_iterator.IsAtEnd(); host_iterator.Advance()) {
     RenderProcessHost* host = host_iterator.GetCurrentValue();
-    if (host->GetBrowserContext() == browser_context)
+    if (host->GetBrowserContext() == browser_context) {
+      // This will also clean up spare RPH references.
       host->ForceReleaseWorkerRefCounts();
+    }
   }
 }