[ServiceWorker] Refactor the codes for releasing the resources for streaming.

[ServiceWorker] Refactor the codes for releasing the resources for streaming in ServiceWorkerURLRequestJob.

As michaeln@ mentioned in https://codereview.chromium.org/832813002/, Kill() may not be called and provider_host_ may be deleted before Kill() is called.
So this change
 - adds streaming_version_ property to ServiceWorkerURLRequestJob.
 - adds ClearStream() method to releases the resources for streaming.
 - calls ClearStream() from both Kill() and the destructor.

BUG=445775
Committed: https://crrev.com/e9520537ba1d85df2273c80de2c50aea0ab8ee72
Cr-Commit-Position: refs/heads/master@{#310944}

[horo, 2015-01-09 04:58:42]

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

[horo, 2015-01-09 04:58:43]

michaeln@
Could you please review this?

[michaeln, 2015-01-09 22:14:48]

lgtm... thnx!

[horo, 2015-01-10 01:59:52]

The CQ bit was checked by horo@chromium.org

[commit-bot: I haz the power, 2015-01-10 02:00:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/844913002/1

[commit-bot: I haz the power, 2015-01-10 02:40:09]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-10 02:40:56]

Patchset 1 (id:??) landed as
https://crrev.com/e9520537ba1d85df2273c80de2c50aea0ab8ee72
Cr-Commit-Position: refs/heads/master@{#310944}