SyncFS: Add file deletion test for LocalToRemoteSyncer

This change...
- adds test for file deletion case
- handles "active_ancestor_path == path" case on AppendRelativePath
- updates the database at the end of directory creation

BUG=240165
TEST=unit_tests --gtest_filter=LocalToRemoteSyncerTest.*
R=kinuko@chromium.org, tzik@chromium.org

Review URL: https://codereview.chromium.org/96063004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238020 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238020