Fix crash with null ServiceWorkerProviderHost in DidDispatchFetchEvent

Fix crash with null ServiceWorkerProviderHost in DidDispatchFetchEvent

The provider host is a WeakPtr and can be destroyed if the tab was
closed.

BUG=520857
Committed: https://crrev.com/a6c778d2a0525c8891d4f6fbd8cee0b08f087f1b
Cr-Commit-Position: refs/heads/master@{#345568}

[falken, 2015-08-26 06:26:19]

falken@chromium.org changed reviewers:
+ horo@chromium.org

[falken, 2015-08-26 06:26:19]

horo@: PTAL

I'm not totally sure the significance of the SSL checks in unittests. I figured
we should check them in all places so we can catch a regression.

[horo, 2015-08-26 07:43:18]

lgtm.

Thank you!

[falken, 2015-08-26 07:48:32]

The CQ bit was checked by falken@chromium.org

[commit-bot: I haz the power, 2015-08-26 07:48:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1304033004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1304033004/1

[commit-bot: I haz the power, 2015-08-26 08:23:14]

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

[commit-bot: I haz the power, 2015-08-26 08:23:15]

Try jobs failed on following builders:
  win_chromium_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)

[falken, 2015-08-26 08:29:47]

The CQ bit was checked by falken@chromium.org

[commit-bot: I haz the power, 2015-08-26 08:30:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1304033004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1304033004/1

[commit-bot: I haz the power, 2015-08-26 09:06:42]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-26 09:07:31]

Patchset 1 (id:??) landed as
https://crrev.com/a6c778d2a0525c8891d4f6fbd8cee0b08f087f1b
Cr-Commit-Position: refs/heads/master@{#345568}