[ServiceWorker] Fix thread jumping race condition in SWProviderHost.

[ServiceWorker] Fix thread jumping race condition in SWProviderHost.

This is using PostTaskAndReplyWithResult() instead of PostTask,
making sure that the callback is destroyed in the same thread as it
was called in.

BUG=457287
Committed: https://crrev.com/8f5ceb741ae7f126c5acc67c7b28949efebeef24
Cr-Commit-Position: refs/heads/master@{#316198}

[mlamouri (slow - plz ping), 2015-02-12 22:50:54]

mlamouri@chromium.org changed reviewers:
+ michaeln@chromium.org

[michaeln, 2015-02-13 00:53:22]

lgtm, nice that PostTaskAndReplay uses Unretained ptrs in there

[mlamouri (slow - plz ping), 2015-02-13 08:55:56]

The CQ bit was checked by mlamouri@chromium.org

[commit-bot: I haz the power, 2015-02-13 08:56:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/918613003/20001

[commit-bot: I haz the power, 2015-02-13 09:45:54]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-13 09:46:48]

Patchset 2 (id:??) landed as
https://crrev.com/8f5ceb741ae7f126c5acc67c7b28949efebeef24
Cr-Commit-Position: refs/heads/master@{#316198}