Fix delegate check BlimpClientContextImpl::SetDelegate

Fix delegate check BlimpClientContextImpl::SetDelegate

In https://codereview.chromium.org/2463423002/, the check was changed to
DCHECK(!delegate_ && delegate) assuming we don't support clearing the
delegate. But we do
https://cs.chromium.org/chromium/src/chrome/browser/android/blimp/chrome_blimp_client_context_delegate.cc?l=33
so changing it back, and add this to the unit test.

BUG=

Committed: https://crrev.com/67574edeaae89b934ce667a0680d178f924df26a
Cr-Commit-Position: refs/heads/master@{#430943}

[Menglin, 2016-11-09 01:42:02]

The CQ bit was checked by mlliu@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-11-09 01:42:34]

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

[Menglin, 2016-11-09 01:42:35]

mlliu@chromium.org changed reviewers:
+ khushalsagar@chromium.org

[Menglin, 2016-11-09 01:42:35]

Hi Khushal, ptal. Thanks!

Menglin

[Khushal, 2016-11-09 01:49:10]

lgtm

[commit-bot: I haz the power, 2016-11-09 02:42:47]

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

[commit-bot: I haz the power, 2016-11-09 02:42:48]

Dry run: This issue passed the CQ dry run.

[Menglin, 2016-11-09 16:09:43]

The CQ bit was checked by mlliu@chromium.org

[commit-bot: I haz the power, 2016-11-09 16:09:56]

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, 2016-11-09 16:14:08]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-11-09 16:16:22]

Description was changed from

==========
Fix delegate check BlimpClientContextImpl::SetDelegate

In https://codereview.chromium.org/2463423002/, the check was changed to
DCHECK(!delegate_ && delegate) assuming we don't support clearing the
delegate. But we do
https://cs.chromium.org/chromium/src/chrome/browser/android/blimp/chrome_blim...
so changing it back, and add this to the unit test.

BUG=
==========

to

==========
Fix delegate check BlimpClientContextImpl::SetDelegate

In https://codereview.chromium.org/2463423002/, the check was changed to
DCHECK(!delegate_ && delegate) assuming we don't support clearing the
delegate. But we do
https://cs.chromium.org/chromium/src/chrome/browser/android/blimp/chrome_blim...
so changing it back, and add this to the unit test.

BUG=

Committed: https://crrev.com/67574edeaae89b934ce667a0680d178f924df26a
Cr-Commit-Position: refs/heads/master@{#430943}
==========

[commit-bot: I haz the power, 2016-11-09 16:16:25]

Patchset 1 (id:??) landed as
https://crrev.com/67574edeaae89b934ce667a0680d178f924df26a
Cr-Commit-Position: refs/heads/master@{#430943}