Move WeakPtrFactory member to the end of LocalExtensionCache

Move WeakPtrFactory member to the end of LocalExtensionCache

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=asargent@chromium.org

Committed: https://crrev.com/167430259992ef059628ed69dae4e413d696cdcd
Cr-Commit-Position: refs/heads/master@{#311376}

[asargent_no_longer_on_chrome, 2015-01-14 00:41:24]

lgtm

A clang check for this would be great!

[dmichael (off chromium), 2015-01-14 00:42:30]

The CQ bit was checked by dmichael@chromium.org

[commit-bot: I haz the power, 2015-01-14 00:43:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/846153005/1

[commit-bot: I haz the power, 2015-01-14 00:48:08]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-14 00:49:52]

Patchset 1 (id:??) landed as
https://crrev.com/167430259992ef059628ed69dae4e413d696cdcd
Cr-Commit-Position: refs/heads/master@{#311376}