chromeos: Destruct DriveResourceMetadata on the blocking pool
Make DriveResourceMetadata's dtor private, add Destroy().
Add DestroyHelper to drive_file_system_util.h
Add DestroyHelperForTests to drive_test_util.h and replace DeleteDriveCache with it.
After switching to DB storage, DriveResourceMetadata will post tasks to the blocking pool.
Since we'd like to avoid locking the UI thread, we don't want to wait for the completion of tasks on the blocking pool.
It means that DriveResourceMetadata must live longer than any tasks posted on the blocking pool.
To achieve this, we post a task to delete DriveResourceMetadata on the blocking pool like DriveCache.
BUG=147299
TEST=unit_tests
Review URL: https://chromiumcodereview.appspot.com/12706012
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@188818 0039d316-1c4b-4281-b951-d872f2087c98
15 files changed
