[SyncFS] Clean up tracker tree traversal

BUG='./unit_tests --gtest_filter=MetadataDatabaseTest.InitializationTest_SimpleTree'

Review URL: https://codereview.chromium.org/183063003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254127 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254127