SyncFS: Add LocalToRemoteSyncer tests

This change...
- adds LocalToRemoteSyncerTest
- separates FakeDriveUploader into its own file
- fixes MetadataDatabase bugs (LocalToRemoteSyncerTest covers them)

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

Review URL: https://codereview.chromium.org/87853003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237891 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237891