Closing app when pressing back key on Web Page view.

Closing app when pressing back key on Web Page view.

[Issue#]   https://bugs.tizen.org/jira/browse/TT-170
[Problem]  Browser didn't close when pressing back key.
[Cause]    There was a check if back button is enabled which prevent
           from closing app when there was no previous web page.
[Solution] Removed unneeded check.
[Verify]   Run browser, visit some web page, press back button, check
           if browser will close.

Change-Id: I46c29149a1d311679f9b6ef141f6dd71dc335363
Signed-off-by: Maciej Skrzypkowski <m.skrzypkows@samsung.com>