GitLab

This crash is because we have root layer is null and we don't handle it properly.
Root layer is null can happen if a scroll occurs before the root layer is attached.

In this patch, we return SCROLL_IGNORED with kNoScrollingLayer reason, same as what
we did in ScrollBegin.

Bug: 895817
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I9937ae2d494f4466018a3ee9a35e787e5e745bd6
Reviewed-on: https://chromium-review.googlesource.com/c/1289437Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Jianpeng Chao <chaopeng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601345}