fix ewk_view_user_agent_set

fix ewk_view_user_agent_set

Right now, even when setting user agent, this change is not seen by web
pages. In order for the user agent to be considered, we need to load url
with override_user_agent param.

In order to simplify the setting of this flag, this patch also refactor
EWebView GetURL/SetURL. Now they get/return GURL instead of strings

Also, this patch prevents webpage from reloading when a load is in
progress.

Bug: http://web.sec.samsung.net/bugzilla/show_bug.cgi?id=14400

Reviewed by: a1.gomes, j.majnert

Change-Id: Ie170885f10927cdda7afd9d5352e6a24944a2d29
Signed-off-by: Arnaud Renevier <a.renevier@samsung.com>