[SyncFS] Make routines using metadata database async

chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc

Index: chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc
diff --git a/chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc b/chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc
index 917c8374597026eb63cb53aebe7e7c1c6970a9e5..580284ec2214bedc0b349ad29a94acf2850f325d 100644
--- a/chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc
+++ b/chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc
@@ -503,7 +503,7 @@ class DriveBackendSyncTest : public testing::Test,
   }
 
   MetadataDatabase* metadata_database() {
-    return remote_sync_service_->GetMetadataDatabase();
+    return remote_sync_service_->GetMetadataDatabaseForTesting();

[nhiroki, 2014/05/29 04:12:58]

As chatted locally, this wrapper function easily hides the consumption I
mentioned in the previous comment. How about removing getter for metadata
database and accessing it asynchronously?

[peria, 2014/06/02 04:30:28]

Yes, direct calls of MetadataDatabase's methods are dangerous.
However, this getter is very useful for this test class.
I replaced all MetadataDatabase's methods with
PostTaskAndReplyWithResult and added a comment for it.
And I will add DCHECK() to check if MetadataDatabase runs on
worker task runner.

WDYT?

   }
 
  private: