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As with ScrollBarViews, this can just size the thumb extra-large and rely on
ScrollBar::Update() to do the real sizing.  To facilitate this, override
GetInsets() to convert GetContentsBounds() into the same thing as
GetTrackBounds(), then use a FillLayout.

Also reorders functions to match superclass order and access level.

Bug: 1005568
Change-Id: Iad7fed5501adcf301ac66f9390c2545a6e00c827
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824028
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Robert Liao <robliao@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699584}