drive: Simplify TruncateOperation with ResourceEntry::local_id

BUG=None
TEST=unit_tests
R=kinaba@chromium.org

Review URL: https://codereview.chromium.org/59443002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233224 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233224