[Android] Handle a null java side of CVC in context menu

[Android] Handle a null java side of CVC in context menu

We might have a native ContentViewCore but not a java side.  Don't
attempt to show the context menu if this is the case.

BUG=410599
Committed: https://crrev.com/f660c935c53b2b22d0ef70f74c28106282fcb25c
Cr-Commit-Position: refs/heads/master@{#297318}

[David Trainor- moved to gerrit, 2014-09-29 23:01:55]

dtrainor@chromium.org changed reviewers:
+ yfriedman@chromium.org

[David Trainor- moved to gerrit, 2014-09-29 23:01:56]

ptal thanks!

[Yaron, 2014-09-29 23:05:42]

lgtm

[David Trainor- moved to gerrit, 2014-09-29 23:38:05]

The CQ bit was checked by dtrainor@chromium.org

[commit-bot: I haz the power, 2014-09-29 23:39:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/610913004/1

[commit-bot: I haz the power, 2014-09-30 00:44:40]

Committed patchset #1 (id:1) as 2e98c957171ee9f46d09cfd401a0ac7e9697b14d

[commit-bot: I haz the power, 2014-09-30 00:45:27]

Patchset 1 (id:??) landed as
https://crrev.com/f660c935c53b2b22d0ef70f74c28106282fcb25c
Cr-Commit-Position: refs/heads/master@{#297318}