Fix wrong grid layout caused by not initialized offset.

When we append non-cached items to grid,  the first visible item
created has a offset of 0. Then we prepend items with cache,  which
will put the first prepended item at the same location of current
first visible item.

Bug 18963829

Change-Id: I174c995ff05acc8492c938bb67a7d4b993629272
2 files changed