Revert of Unload the ScopedGaiaAuthExtension asynchronously.

Revert of Unload the ScopedGaiaAuthExtension asynchronously. (patchset #2 id:20001 of https://codereview.chromium.org/961443003/)

Reason for revert:
Broke BookmarkSyncPromoControllerTest.SignInLink on Mac ASan 64: https://build.chromium.org/p/chromium.memory/builders/Mac%20ASan%2064%20Tests%20%281%29/builds/7363

Original issue's description:
> Unload the ScopedGaiaAuthExtension asynchronously.
>
> If we don't do this, then we have a race condition between unloading
> this extension, and trying to use it because we have manually
> navigated to it.
>
> BUG=460431
>
> Committed: https://crrev.com/1805c4c5c4b5292cca51a54f379cde498db7238b
> Cr-Commit-Position: refs/heads/master@{#318140}

TBR=xiyuan@chromium.org,noms@google.com,noms@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=460431
Committed: https://crrev.com/7f4aed4ac87717e339303e2c494be4855cab738d
Cr-Commit-Position: refs/heads/master@{#318176}

[Sam McNally, 2015-02-26 02:36:41]

Created Revert of Unload the ScopedGaiaAuthExtension asynchronously.

[commit-bot: I haz the power, 2015-02-26 02:37:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/944923007/1

[commit-bot: I haz the power, 2015-02-26 02:42:12]

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

[commit-bot: I haz the power, 2015-02-26 02:42:13]

Failed to commit the patch.

[commit-bot: I haz the power, 2015-02-26 02:42:34]

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

[Sam McNally, 2015-02-26 02:57:26]

The CQ bit was checked by sammc@chromium.org

[commit-bot: I haz the power, 2015-02-26 02:58:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/944923007/1

[commit-bot: I haz the power, 2015-02-26 02:58:48]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-26 02:59:21]

Patchset 1 (id:??) landed as
https://crrev.com/7f4aed4ac87717e339303e2c494be4855cab738d
Cr-Commit-Position: refs/heads/master@{#318176}