Support calling BackgroundSyncProviderThreadProxy::GetThreadInstance after the worker has stopped

Support calling BackgroundSyncProviderThreadProxy::GetThreadInstance after the worker has stopped

This fixes the crash introduced by https://codereview.chromium.org/1290073003/

Crashes: http://test-results.appspot.com/dashboards/flakiness_dashboard.html#showExpectations=true&tests=http%2Ftests%2Fcache%2Fcached-main-resource.html

BUG=502214
Committed: https://crrev.com/0b5b16d3a9ac125e720843a4a2fd1dd593e5751b
Cr-Commit-Position: refs/heads/master@{#343953}

[jkarlin, 2015-08-18 14:57:08]

jkarlin@chromium.org changed reviewers:
+ jsbell@chromium.org

[jkarlin, 2015-08-18 14:58:23]

jsbell: PTAL at this additional crash fix.

[jsbell, 2015-08-18 16:29:00]

lgtm

[jkarlin, 2015-08-18 17:40:52]

The CQ bit was checked by jkarlin@chromium.org

[commit-bot: I haz the power, 2015-08-18 17:41:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1302543002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1302543002/1

[commit-bot: I haz the power, 2015-08-18 17:45:58]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-18 17:46:41]

Patchset 1 (id:??) landed as
https://crrev.com/0b5b16d3a9ac125e720843a4a2fd1dd593e5751b
Cr-Commit-Position: refs/heads/master@{#343953}