CC: Fix call order of ScrollOffsetDelegate in ActivatePendingTree.

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

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226255 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226255