SyncFS: Add RemoteToLocalSyncer tests for conflict cases

This change...
- fixes folder-folder conflict handling.
- enhances FakeRemoteChangeProcessor to specify local metadata and file changes.
- adds tests for conflict cases except file-file conflict case.


BUG=240165
TEST=unit_tests --gtest_filter=RemoteToLocalSyncerTest.*

Review URL: https://codereview.chromium.org/87853002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237326 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237326