Make FileStream::Seek async and add FileStream::SeekSync for sync operation

net/disk_cache/entry_unittest.cc

Index: net/disk_cache/entry_unittest.cc
diff --git a/net/disk_cache/entry_unittest.cc b/net/disk_cache/entry_unittest.cc
index 21a5bb1548ba32aa8648a5312cc6bc00eca013d2..8168ae8e4c10ca54b7a4fa0f79c5727b17c2bcf9 100644
--- a/net/disk_cache/entry_unittest.cc
+++ b/net/disk_cache/entry_unittest.cc
@@ -1786,7 +1786,7 @@ TEST_F(DiskCacheEntryTest, MemoryOnlyDoomSparseEntry) {
 // The way an CompletionCallback works means that all tasks (even new ones)
 // are executed by the message loop before returning to the caller so the only
 // way to simulate a race is to execute what we want on the callback.
-class SparseTestCompletionCallback: public TestCompletionCallbackBase {
+class SparseTestCompletionCallback: public net::TestCompletionCallbackBase {
  public:
   explicit SparseTestCompletionCallback(disk_cache::Backend* cache)
       : cache_(cache),