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When a user clicks and drags the thumb, the sequence of events that
gets generated is pointerdown, pointermoves(s), pointerup. The very
first pointerdown sets up the state necessary for the scrollbar thumb
drag. The pointerdown also injects a synthetic GestureScrollBegin in
the CompositorThreadEventQueue. However, this GSB won't get dispatched
until the next VSync. Sometimes, a pointerdown and the following
pointermove can arrive at the ScrollbarController before the GSB is
dispatched at the VSync. When this happens, the value of the
currently_scrolling_node is null (since the GSB wasn't dispatched yet).
This in turn causes the drag_state_ to be nullopt and we get stuck in
a state where thumb drag doesn't work (because we keep early'ing out in
ScrollbarController::HandlePointerMove).

The fix here is to use ScrollbarLayerImplBase::scroll_element_id() to
look up the target node. That way, even tough the pointermove arrives
before the GSB, the node to be scrolled can still be queried by
ScrollbarController::HandlePointerMove.

Bug: 1057929
Change-Id: I5cd584b9298c97fc2feb058aa6b46d264f0cdc17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2085533
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747411}