GitLab

This patch refactors FindScrollLayerForDeviceViewportPoint as
FindScrollNodeForDeviceViewportPoint which both simplifies code and lets
us remove two users of ScrollNode's owning_layer_id.

BUG=693740
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2824683002
Cr-Commit-Position: refs/heads/master@{#465677}