[M120 Migration] Possible fix for crash during shutdown insideSelectionControllerEfl...

[M120 Migration] Possible fix for crash during shutdown insideSelectionControllerEfl::CancelContextMenu

In pre-aura, WebContentViewEfl was owned by RWHVEfl, so the order of destruction
was guaranteed. But in post-aura, WCVAHelper is owned by WCVA, so during shutdown,
sometimes due to destruction sync issue, WCVAHelper may not be a valid pointer
in SelectionControllerEfl (which is a member of RWHVHelper). So instead of using
WCVAHelper, use WebContentsEflDelegate directly to show and cancel context menu in
SelectionControllerEfl class.

Reference:
https://review.tizen.org/gerrit/295842

Change-Id: Ic01aa40000567e8605af47a46163462e3196e57e