fix build after clang roll

fix build after clang roll

../../chrome/browser/sync_file_system/drive_backend/sync_engine.cc:33:21:error:
field 'weak_ptr_factory_' is uninitialized when used here
[-Werror,-Wuninitialized]
      task_manager_(weak_ptr_factory_.GetWeakPtr()),
                    ^

This is wrong as WeakPtrFactory should be last, but let's fix the build for now
(and the code as-is is broken too, so it's not making things worse).

This was broken by https://codereview.chromium.org/26594002/

BUG=303818, 290204
TBR=dmichael@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=228322

[Nico, 2013-10-12 06:37:07]

Committed patchset #1 manually as r228322.

[dmichael (off chromium), 2013-10-12 13:24:36]

That's a nice new warning. This is not the only one I messed up tearing through
these, but others got caught by tests.

Thanks, lgtm

[kinuko, 2013-10-15 03:13:35]

(cc-ing tzik)