User agent set depending on browsing mode (desktop/mobile).

User agent set depending on browsing mode (desktop/mobile).

[Issue#]  https://bugs.tizen.org/jira/browse/TT-132
[Problem] Browising mode switching was not implemented.
[Cause]   N/A
[Solution]Added changing user agent by using EWK API
          method: ewk_view_user_agent_set.
[Verify]  Open new page, go to google.com then
          settings > view mobile web then close settings,
          type in new URL google.com, it should appear
          in mobile mode.

Change-Id: I8a6888d09b94f97eb51b9d2440f04bf8e2ba544c
Signed-off-by: Maciej Skrzypkowski <m.skrzypkows@samsung.com>