CompositingInputsUpdater should compute scrollParent

Instead of walking up the tree to find the scroll parent 
for every RenderLayer when computing the compositing
requirements, we now compute it while walking down the tree
for only the RenderLayers with dirty compositing state.

Note: This CL is a minor tweak of abarth's patch
(https://codereview.chromium.org/340303004). It adds a
small walk to ensure that there is no stacking context
between the layer and the scroller.

BUG=None

Review URL: https://codereview.chromium.org/344023002

git-svn-id: svn://svn.chromium.org/blink/trunk@176595 bbb929c8-8fbe-4397-9dbb-9b2b20218538