drive: Manage SyncClient tasks with one map

Managing 3 std::set is dificult.
Remove tasks will be also managed in this way.

BUG=260537
TEST=unit_tests
R=kinaba@chromium.org

Review URL: https://codereview.chromium.org/90733002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237549 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237549