WebViewCore::createPrerenderCanvas failed unexpectedly

m_scrollSetTime not in WebViewCore's ctor makes it randomly
initialized, which leads to spurious scroll when creating
prerender in WebViewCore::createPrerenderCanvas, which hence
fails the creation.

Fix it by adding m_scrollSetTime in initialization list.

Change-Id: I4d834cfae091f645ac9cb4c99a4394164772872c
Signed-off-by: Yuyang Du <yuyang.du@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Signed-off-by: Beare, Bruce J <bruce.j.beare@intel.com>
Signed-off-by: Yuyang Du <yuyang.du@intel.com>