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There were two subtle bugs in the original implementation:

i. cc::ScrollTimeline was incorrectly initialized with active_id_ ==
   pending_id_. This is incorrect as the current active tree may not
   yet have the relevant ScrollNode. Instead only pending_id_ should be
   set, and it will be promoted to active via the existing logic when
   the pending tree activates.

ii. cc::ScrollTimeline::PushPropertiesTo incorrectly updated the impl
    thread active_id_ from the main side. This is incorrect, main never
    has a valid active_id_ value (since it never gets updated by pending
    tree activations). This should only push the pending_id_ value,
    which may change e.g. if the scroller went display:none and back.

Bug: 853231
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux-blink-gen-property-trees;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ib7bf4e82e779d11895c373f7c786727c1bcadb9d
Reviewed-on: https://chromium-review.googlesource.com/1110438
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569790}