Clear contexts on EWebView exit

Clear contexts on EWebView exit

When user turns on a private browsing mode, we create a new context
and save an old one. If user wants to close a browser under these
circumstance (with private browsing turned on) we have two contexts open
where the default one is unused (default context should be removed
as the last one). In order to work properly we need to delete a temporarily
created context for a private browsing purpose and bring back the default
one.

Original beta/m42 patch:
http://165.213.202.130/gerrit/#/c/87523/

Bug: http://107.108.218.239/bugzilla/show_bug.cgi?id=13557
Reviewed by: a1.gomes, djmix.kim

Change-Id: I6dd1391f9bbbb800f9149a94bf296a4ae7642a22
Signed-off-by: Daniel Waślicki <d.waslicki@samsung.com>