Fix WeakPtrFactory member in SSOAccessTokenFetcher

Fix WeakPtrFactory member in SSOAccessTokenFetcher

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=rogerta@chromium.org

Committed: https://crrev.com/19e69d46ae633294f50a3f59cc58fd142d60a370
Cr-Commit-Position: refs/heads/master@{#311408}

[Roger Tawa OOO till Jul 10th, 2015-01-14 01:34:51]

lgtm

For my own info, this rule about being last breaks if some class derives from a
class that contains a weak pointer factory, is that correct?

[dmichael (off chromium), 2015-01-14 03:11:34]

On 2015/01/14 01:34:51, Roger Tawa wrote:
> lgtm
> 
> For my own info, this rule about being last breaks if some class derives from
a
> class that contains a weak pointer factory, is that correct?
That's right. I haven't made the check smart enough for that, though we could.
Thanks for the review!

[dmichael (off chromium), 2015-01-14 03:11:37]

The CQ bit was checked by dmichael@chromium.org

[commit-bot: I haz the power, 2015-01-14 03:12:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/827093006/1

[commit-bot: I haz the power, 2015-01-14 04:07:30]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-14 04:08:40]

Patchset 1 (id:??) landed as
https://crrev.com/19e69d46ae633294f50a3f59cc58fd142d60a370
Cr-Commit-Position: refs/heads/master@{#311408}