Flush on completion of copying.

The COPY_SYNC_OPTION_SYNC flag was missing when registering FSP file systems.
As a result, when copying between file systems the last chunk could be lost.

TEST=Tested manually in Files app.
BUG=439830

Review URL: https://codereview.chromium.org/778103003

Cr-Commit-Position: refs/heads/master@{#307219}

2 files changed, 7 insertions, 4 deletions

diff --git a/storage/browser/fileapi/external_mount_points.cc b/storage/browser/fileapi/external_mount_points.cc
index 07df4f4..04b715f 100644
--- a/storage/browser/fileapi/external_mount_points.cc
+++ b/storage/browser/fileapi/external_mount_points.cc
@@ -96,10 +96,6 @@ bool ExternalMountPoints::RegisterFileSystem(
     FileSystemType type,
     const FileSystemMountOption& mount_option,
     const base::FilePath& path_in) {
-  // COPY_SYNC_OPTION_SYNC is only applicable to native local file system.
-  DCHECK(type == kFileSystemTypeNativeLocal ||
-         mount_option.copy_sync_option() != COPY_SYNC_OPTION_SYNC);
-
   base::AutoLock locker(lock_);
 
   base::FilePath path = NormalizeFilePath(path_in);
diff --git a/storage/common/fileapi/file_system_mount_option.h b/storage/common/fileapi/file_system_mount_option.h
index dc5cdc4..38daa96 100644
--- a/storage/common/fileapi/file_system_mount_option.h
+++ b/storage/common/fileapi/file_system_mount_option.h
@@ -9,7 +9,14 @@ namespace storage {
 
 // Option for specifying if disk sync operation is wanted after copying.
 enum CopySyncOption {
+  // No syncing is required after an operation is completed.
   COPY_SYNC_OPTION_NO_SYNC,
+
+  // Syncing is required in order to commit written data. Note, that syncing
+  // is only invoked via FileStreamWriter::Flush() and via base::File::Flush()
+  // for native files. Hence, syncing will not be performed for copying within
+  // non-native file systems as well as for non-native copies performed with
+  // snapshots.
   COPY_SYNC_OPTION_SYNC,
 };