[SyncFileSystem] Push metadata with empty MD5 for to_be_fetched file.

Populating this for not-yet-fetched file causes NOT_MODIFIED case on downloading the file.

BUG=229764

Review URL: https://chromiumcodereview.appspot.com/16152003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@203097 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/sync_file_system/drive_file_sync_service.cc

@@ -770,7 +770,7 @@ void DriveFileSyncService::DidGetDirectoryContentForBatchSync(
     // Save to be fetched file to DB for restore in case of crash.
     DriveMetadata metadata;
     metadata.set_resource_id(entry.resource_id());
-    metadata.set_md5_checksum(entry.file_md5());
+    metadata.set_md5_checksum(std::string());
     metadata.set_conflicted(false);
     metadata.set_to_be_fetched(true);