Flush socket pools and SPDY session pool properly on explicit requests and network changes.

net/base/host_resolver.h

Index: net/base/host_resolver.h
diff --git a/net/base/host_resolver.h b/net/base/host_resolver.h
index 8f7bd097bf36690eef03495516f32677eadff97b..a65bc6b6818f1353c2ec10600e58f2034553c003 100644
--- a/net/base/host_resolver.h
+++ b/net/base/host_resolver.h
@@ -156,6 +156,9 @@ class HostResolver : public base::RefCountedThreadSafe<HostResolver> {
   // Unregisters an observer previously added by AddObserver().
   virtual void RemoveObserver(Observer* observer) = 0;
 
+  // Gives the HostResolver an opportunity to flush state.
+  virtual void Flush() {}
+
   // Sets the default AddressFamily to use when requests have left it
   // unspecified. For example, this could be used to restrict resolution
   // results to AF_INET by passing in ADDRESS_FAMILY_IPV4, or to