[Cronet] Check whether request is done before spinning up message loop

[Cronet] Check whether request is done before spinning up message loop

When client writes to OutputStream, the request might have already completed.
If that is the case, do not spin up the message loop since that will block
the calling thread forever.

This CL also adds tests.

TBR=mef@chromium.org
BUG=626653
NOTRY=true
NOPRESUBMIT=true
Review-Url: https://codereview.chromium.org/2131323002
Cr-Commit-Position: refs/heads/master@{#404652}
(cherry picked from commit b08d71cee8ab485bc4bf6872d01be9fe8b4df296)

[xunjieli, 2016-07-11 17:45:16]

Hi Misha, this is to merge the change into M53. I will verify locally that the
tests are still passing.

[mef, 2016-07-11 17:49:38]

lgtm

[xunjieli, 2016-07-11 18:40:48]

On 2016/07/11 17:49:38, mef wrote:
> lgtm

Thanks! I verified locally that Cronet tests are still passing.

[xunjieli, 2016-07-11 18:40:53]

The CQ bit was checked by xunjieli@chromium.org

[commit-bot: I haz the power, 2016-07-11 18:41:20]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-07-11 18:43:13]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-07-11 18:43:18]

CQ bit was unchecked.