[WRTjs] Seperate runtime dependency of XWalkExtension

[WRTjs] Seperate runtime dependency of XWalkExtension

This patch refactors to remove WRT dependency from XWalkExtension
classes to make a common module for other runtimes. Specially,
this is for Hbb runtime by ewk interface later.

Delegation classes, named as WRTXwalkExtension* will have roles of
runtime dependent behaviors.

Change-Id: I5a39b2e9cb26708953541b69de8c0830330b3f8e
Signed-off-by: DongHyun Song <dh81.song@samsung.com>