The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/38ec90922bb8f6e9c2b6aaacdc27c13ae6dde8f2

commit 38ec90922bb8f6e9c2b6aaacdc27c13ae6dde8f2
Author: chaopeng <chaopeng@chromium.org>
Date: Mon Jan 16 21:14:23 2017

Return main thread scroll layer FindScrollLayerForDeviceViewportPoint

In FindScrollLayerForDeviceViewportPoint, we return null for main thread scroll
layer. But some calls only use the layer to look up animation controller and
that null return will make it wrong.

In the patch we return a layer without checking it is scroll in main thread.

BUG= 679804 
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2621133002
Cr-Commit-Position: refs/heads/master@{#443943}

[modify] https://crrev.com/38ec90922bb8f6e9c2b6aaacdc27c13ae6dde8f2/cc/trees/layer_tree_host_impl.cc
[modify] https://crrev.com/38ec90922bb8f6e9c2b6aaacdc27c13ae6dde8f2/cc/trees/layer_tree_host_impl_unittest.cc