HostResolver: don't interpret NULL callback argument as a request to do synchronous resolution.

net/proxy/sync_host_resolver_bridge_unittest.cc

Index: net/proxy/sync_host_resolver_bridge_unittest.cc
diff --git a/net/proxy/sync_host_resolver_bridge_unittest.cc b/net/proxy/sync_host_resolver_bridge_unittest.cc
index 1459a1eb18c7aba6d0c29cc1e9678ffa59d147af..75f7f16e25b7ad5fc8f67cb8d4feb3c9468c0a2a 100644
--- a/net/proxy/sync_host_resolver_bridge_unittest.cc
+++ b/net/proxy/sync_host_resolver_bridge_unittest.cc
@@ -47,6 +47,13 @@ class BlockableHostResolver : public HostResolver {
     return ERR_IO_PENDING;
   }
 
+  virtual int ResolveFromCache(const RequestInfo& info,
+                               AddressList* addresses,
+                               const BoundNetLog& net_log) OVERRIDE {
+    NOTIMPLEMENTED();
+    return ERR_UNEXPECTED;
+  }
+
   virtual void CancelRequest(RequestHandle req) OVERRIDE {
     EXPECT_EQ(reinterpret_cast<RequestHandle*>(1), req);
     was_request_cancelled_ = true;