[Chromoting] Share code that updates the UI when clientSession changes.

[Chromoting] Share code that updates the UI when clientSession changes.

This moves the duplicated code into a shared function and adds checks
to ensure the UI element exists before calling setClientSession on it.

NOTRY=True
BUG=

Committed: https://crrev.com/b3352ce8ec00ec703fd5531ed282fe7f3674467b
Cr-Commit-Position: refs/heads/master@{#306741}

[garykac, 2014-12-03 01:49:29]

garykac@chromium.org changed reviewers:
+ jamiewalch@chromium.org

[Jamie, 2014-12-04 01:44:04]

lgtm

[garykac, 2014-12-04 01:49:26]

The CQ bit was checked by garykac@chromium.org

[commit-bot: I haz the power, 2014-12-04 01:49:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/767013007/1

[commit-bot: I haz the power, 2014-12-04 01:50:33]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-04 01:51:24]

Patchset 1 (id:??) landed as
https://crrev.com/b3352ce8ec00ec703fd5531ed282fe7f3674467b
Cr-Commit-Position: refs/heads/master@{#306741}