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Element::scrollIntoViewIfNeeded computes the scroll bounding box by calling containerNode::boundingBox(), for performing autoscroll.
This bounding box rect is passed to RenderLayerScrollableArea::exposeRect(), where actual
scroll parent rect to be scrolled is computed.

This bound is computed by using two factors.
current Scroll Offset and the target Scroll Offset.

target scroll offset is Y position of the element which has to be
scrolled into view. This Y position computation includes top border
of the parent container.
for example. in the bug reported page.
Parent grid has border of 5 px so,
the position of first  child would be 5  and
the position of second child would be 55 and so on

But [current]scroll offset is independent of top border.
When target bounding box is computed, target scroll offset is
substracted from current scroll offset which results in incorrect
scroll offset.
for example. in the bug reported page
1. When page loads grid is scrolled to 200.
2. Parents top border is      5,
3. Position of 2nd child is  55
4. so the target scroll offset is computed as,
   55 - 200 = -145 , which is incorrect &
   should be,  50 - 200 = -150
5. Hence the borderTop included in the position causes the issue, and needs to be removed from computed scroll offset value.

BUG=295848

Review URL: https://codereview.chromium.org/568453002

git-svn-id: svn://svn.chromium.org/blink/trunk@183917 bbb929c8-8fbe-4397-9dbb-9b2b20218538