Changed implementation of EWebView::GetSnapshot

Changed implementation of EWebView::GetSnapshot

Previously snapshot was obtained from BackingStore, which is not a valid
approach any more. In this patch, the implemenation is changed so as to
get snapshot directly from GPU buffer via RenderViewHostViewEfl.

From EWK perspective, this fixes UTC ewk_view_screenshot_contents_get fail,
and the ewk side patch http://suprem.sec.samsung.net/gerrit/#/c/2214/ should
be merged after landing this patch.

Issue: CBWEBVIEW-526
Change-Id: I010375946983685be2e97b1bd46d5b0db2ccdcd5

Conflicts:

	impl/eweb_view.cc