Fix WeakPtrFactory member in LoginDisplayHostImpl

Fix WeakPtrFactory member in LoginDisplayHostImpl

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=stevenjb@chromium.org

Committed: https://crrev.com/51b7efebea361ee9a0bcff532470ffe61335ea2d
Cr-Commit-Position: refs/heads/master@{#311373}

[stevenjb, 2015-01-14 00:25:56]

lgtm

[dmichael (off chromium), 2015-01-14 00:34:09]

The CQ bit was checked by dmichael@chromium.org

[commit-bot: I haz the power, 2015-01-14 00:35:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/841103003/1

[commit-bot: I haz the power, 2015-01-14 00:42:59]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-14 00:44:35]

Patchset 1 (id:??) landed as
https://crrev.com/51b7efebea361ee9a0bcff532470ffe61335ea2d
Cr-Commit-Position: refs/heads/master@{#311373}