Fix crash when opening native messaging channel.

Fix crash when opening native messaging channel.

RenderWidgetHost::GetView() may return NULL, e.g. if the renderer has
crashed. MessageService::OpenChannelToNativeApp() wasn't handling this
case properly.

BUG=374778
Committed: https://crrev.com/41d870461176a8f76402a64ea4c1a4bf00275178
Cr-Commit-Position: refs/heads/master@{#310160}

[Sergey Ulanov, 2015-01-06 18:09:38]

sergeyu@chromium.org changed reviewers:
+ kalman@chromium.org

[not at google - send to devlin, 2015-01-06 20:37:03]

lgtm

[Sergey Ulanov, 2015-01-06 21:23:29]

The CQ bit was checked by sergeyu@chromium.org

[commit-bot: I haz the power, 2015-01-06 21:23:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/787253009/1

[commit-bot: I haz the power, 2015-01-06 22:30:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-06 22:31:43]

Patchset 1 (id:??) landed as
https://crrev.com/41d870461176a8f76402a64ea4c1a4bf00275178
Cr-Commit-Position: refs/heads/master@{#310160}