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When the track rect or scaled track rect is empty, UiResources of Track
and Thumb resources are released, instead of holding in the memory.
These UIResources will be created on PaintedScrollbarLayer::Update when
the Track rect is not empty. Similary When Scrollbar doesnt have Thumb
release UIResources of Thumb.

Bug=410845

Review URL: https://codereview.chromium.org/537943003

Cr-Commit-Position: refs/heads/master@{#295515}