drive: Rewrite UpdateOpreation to access cache/metadata on the blocking pool

chrome/browser/chromeos/drive/file_cache_unittest.cc

Index: chrome/browser/chromeos/drive/file_cache_unittest.cc
diff --git a/chrome/browser/chromeos/drive/file_cache_unittest.cc b/chrome/browser/chromeos/drive/file_cache_unittest.cc
index 7e4a8bba9f190ddeeb0823d79ee55444a0191b05..5a63b0daf75f20ab0e353fc32e6d90e014634190 100644
--- a/chrome/browser/chromeos/drive/file_cache_unittest.cc
+++ b/chrome/browser/chromeos/drive/file_cache_unittest.cc
@@ -336,8 +336,12 @@ class FileCacheTest : public testing::Test {
     expected_sub_dir_type_ = expected_sub_dir_type;
 
     FileError error = FILE_ERROR_OK;
-    cache_->ClearDirtyOnUIThread(
-        resource_id, md5,
+    PostTaskAndReplyWithResult(
+        blocking_task_runner_,
+        FROM_HERE,
+        base::Bind(&FileCache::ClearDirty,
+                   base::Unretained(cache_.get()),
+                   resource_id, md5),
         google_apis::test_util::CreateCopyResultCallback(&error));
     google_apis::test_util::RunBlockingPoolTask();
     VerifyCacheFileState(error, resource_id, md5);