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There's a bug where the thumb in the Cocoa overlay
scrollbar becomes more narrow than the track width.

This bug is caused by
BaseScrollBarThumb::SetLength() which sets the
thumb's size according to its preferred size. Since
the CocoaScrollBarThumb::CalculatePreferredSize()
returns the thumb's minimum's size, SetLength()
would set the thumb's size to be too small.

This CL fixes this issue by returning the thumb's
current thickness instead of the minimum size in
CocoaScrollBarThumb::CalculatePreferredSize().

Additionally, this CL changes the logic so that the
thumb and track bounds and size won't be insetted.
Instead, the thumb's painting will be inset in the
OnPaint() function.

The kExpandedScrollbarThumbInset value is also
removed.

Bug:682466

Change-Id: I59411650df8acfb4e628efbb1a9f0646c2ce385b
Reviewed-on: https://chromium-review.googlesource.com/741039
Commit-Queue: Sarah Chan <spqchan@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513047}