Flush the SimpleBackendImpl's SWP in tests.

net/disk_cache/simple/simple_backend_impl.h

Index: net/disk_cache/simple/simple_backend_impl.h
diff --git a/net/disk_cache/simple/simple_backend_impl.h b/net/disk_cache/simple/simple_backend_impl.h
index 4f01351752e1bd80893966732bca1a5f5d448f15..61c80f9b43f63c68edb2a2b82b4277d0860974d9 100644
--- a/net/disk_cache/simple/simple_backend_impl.h
+++ b/net/disk_cache/simple/simple_backend_impl.h
@@ -64,6 +64,9 @@ class NET_EXPORT_PRIVATE SimpleBackendImpl : public Backend,
   // operations to construct a new object.
   void OnDeactivated(const SimpleEntryImpl* entry);
 
+  // Flush our SequencedWorkerPool.
+  static void FlushWorkerPoolForTesting();
+
   // Backend:
   virtual net::CacheType GetCacheType() const OVERRIDE;
   virtual int32 GetEntryCount() const OVERRIDE;