Cherry-pick: CC: Fix call order of ScrollOffsetDelegate in ActivatePendingTree.
primiano@chromium.org authored
Cherry-pick of chromium crrev.com/25496002

BUG: 10959756

Original description:

When a full tree synchronization happens, the ScrollOffsetDelegate
is called too early (i.e. before DidBecomeActive()) and sees always
a 0x0 scroll offset.
This CL swaps the call order of SetRootLayerScrollOffsetDelegate
and DidBecomeActive during the active tree activation in
LayerTreeHostImpl::ActivatePendingTree.
Updating also the unit tests to prevent future regressions
on the RootLayerScrollOffsetDelegate.

BUG=b/10959756
TBR=mkosiba@chromium.org

Review URL: https://codereview.chromium.org/25496002

Change-Id: Ib95bdb8403ae367490550dea0963a166cbc5a428
331ba8b1