Allow dynamic change to WebRTC's multiple_routes_enabled preference.

Allow dynamic change to WebRTC's multiple_routes_enabled preference.

Currently, this preference is not monitored such that any change through the extension api will require a restart of chrome.

landed fix for crbug.com/457629 is to expose extension api to allow this be changed dynamically.

Tested with the extension.

BUG=466197
Committed: https://crrev.com/3e1269e6e3950185ff3cc791faff3781acc24064
Cr-Commit-Position: refs/heads/master@{#320134}

[guoweis_left_chromium, 2015-03-11 17:35:02]

guoweis@chromium.org changed reviewers:
+ juberti@chromium.org, thakis@chromium.org

[guoweis_left_chromium, 2015-03-11 17:35:22]

guoweis@chromium.org changed reviewers:
+ battre@chromium.org
- thakis@chromium.org

[guoweis_left_chromium, 2015-03-11 17:35:22]

PTAL.

[battre, 2015-03-11 17:44:41]

lgtm

[guoweis_left_chromium, 2015-03-11 18:04:29]

The CQ bit was checked by guoweis@chromium.org

[guoweis_left_chromium, 2015-03-11 18:04:29]

The patchset sent to the CQ was uploaded after l-g-t-m from battre@chromium.org
Link to the patchset: https://codereview.chromium.org/999833002/#ps20001 (title:
" ")

[commit-bot: I haz the power, 2015-03-11 18:15:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/999833002/20001

[guoweis_left_chromium, 2015-03-11 20:22:40]

The CQ bit was checked by guoweis@chromium.org

[guoweis_left_chromium, 2015-03-11 20:22:40]

The patchset sent to the CQ was uploaded after l-g-t-m from battre@chromium.org
Link to the patchset: https://codereview.chromium.org/999833002/#ps20001 (title:
" ")

[commit-bot: I haz the power, 2015-03-11 20:23:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/999833002/20001

[commit-bot: I haz the power, 2015-03-11 20:24:33]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-11 20:26:04]

Patchset 2 (id:??) landed as
https://crrev.com/3e1269e6e3950185ff3cc791faff3781acc24064
Cr-Commit-Position: refs/heads/master@{#320134}