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This cl adds additional tracing for smooth scrolling path in cc. A
NOTREACHED is also added to the invalid case that the scrolling node
exists in layer tree host impl, but the animation update fails.

Bug: 797708
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: If17af64781e78d02f09433e1c0e479231e69b4c2
Reviewed-on: https://chromium-review.googlesource.com/1145736Reviewed-by: weiliangc <weiliangc@chromium.org>
Commit-Queue: Sahel Sharify <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579069}