Move caret to horizontal boundary on Linux.

Make shouldMoveCaretToHorizontalBoundaryWhenPastTopOrBottom
return true on Linux. This brings the behavior when clicking
below a block of text in line with other Linux applications:
rather than caret being horizontally-aligned with the
pointer, it is positioned at the end of the text.

BUG=20029

Review URL: https://codereview.chromium.org/224243002

git-svn-id: svn://svn.chromium.org/blink/trunk@171718 bbb929c8-8fbe-4397-9dbb-9b2b20218538