SyncFS: Implement FindTrackerByParentAndFileIDForUpload


BUG=237326
TEST=n/a (will be covered in a following CL)
NOTRY=true

Review URL: https://codereview.chromium.org/90673002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237542 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237542