Use layout units in PinchViewport::scrollIntoView.

The pinch viewport visibleRect is snapped to LayoutRect in
RenderBox::scrollRectToVisible, but it remained at full precision
FloatRect when used in PinchViewport::scrollIntoView.  This caused a
small scroll in cases that should've been a no-op.

This changes PinchViewport::scrollIntoView to cast
visibleRect() to a LayoutRect before using it, fixing the issue by
making the imprecision cancel out.  I also changed the argument rect to
a LayoutRect to avoid the reverse problem of too much precision in that
one in the future.

NOTRY=true
BUG=428826

Review URL: https://codereview.chromium.org/678153003

git-svn-id: svn://svn.chromium.org/blink/trunk@184818 bbb929c8-8fbe-4397-9dbb-9b2b20218538