Move WeakPtrFactory to the end of AsyncHandleWaiter

Move WeakPtrFactory to the end of AsyncHandleWaiter

This forces WeakPtrs to be invalidated before other members' destructors run.
See the bug or the documentation of WeakPtrFactory for more information. I'm
in the process of trying to turn on a clang check for this (see the bug).

BUG=303818
R=morrita@chromium.org

Committed: https://crrev.com/e8841f47577c6c1b9eb7dc501762b5f2487bee42
Cr-Commit-Position: refs/heads/master@{#311349}

[Hajime Morrita, 2015-01-13 18:31:45]

lgtm. I did it again so having Clang to check it would be great.

[Hajime Morrita, 2015-01-13 18:31:51]

The CQ bit was checked by morrita@chromium.org

[commit-bot: I haz the power, 2015-01-13 18:32:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/848533003/1

[commit-bot: I haz the power, 2015-01-13 19:08:03]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-01-13 19:08:04]

Try jobs failed on following builders:
  ios_dbg_simulator on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)

[dmichael (off chromium), 2015-01-13 21:24:20]

The CQ bit was checked by dmichael@chromium.org

[commit-bot: I haz the power, 2015-01-13 21:25:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/848533003/1

[commit-bot: I haz the power, 2015-01-13 22:32:12]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-13 22:33:05]

Patchset 1 (id:??) landed as
https://crrev.com/e8841f47577c6c1b9eb7dc501762b5f2487bee42
Cr-Commit-Position: refs/heads/master@{#311349}