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This CL improves readability by splitting GetScrollDeltaFromPointerDown
into 2 separate functions with specific tasks:
- GetScrollbarPartFromPointerDown: Determines the ScrollbarPart that
was hit based on the supplied pointer position.
- GetScrollOffsetForScrollbarPart: Determines ScrollOffset based on the
hit tested ScrollbarPart and the orientation.

Bug: 987163
Change-Id: I89acddc1dd8d65042c468383a9e99abd50e4b7ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1716606Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#680449}