SyncFS: Fix MetadataDatabase::RemoveTracker

Current RemoveTracker does not remove the given tracker from dirty trackers
set and descendant trackers. This makes it remove all of them, and adds tests
for deletion handling.


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

Review URL: https://codereview.chromium.org/85273002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237232 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237232