overflow-y: hidden areas can still be scrolled vertically

Previously, if a say div has an overflown content on 'y' but is
styled as "overflow-x: auto; overflow-y: hidden", RenderBox::canBeProgramaticallyScrolled
would still return true. It interfers, among other things, with the way
autoscroll works.

Patch fixes it by adding two helper methods to RenderBox class in order to verify a box'
scrollability in a given axis (x or y); They are used when checking if a given box is in
fact programatically scrollable. Hence RenderBox::canBeScrolledAndHasScrollableArea logic
gets hardened.

Blink autoscroll behavior now matches Sarafi nightlies, Firefox 23 and Opera12 (pre-blink)
in that sense.

Patch also adds a test to ensure autoscrolling only happens on a given
axis if it is scrollable in that direction, according to its style.

Corresponding WebKit revision: https://trac.webkit.org/changeset/154383

BUG=chromium:286345

Review URL: https://chromiumcodereview.appspot.com/23567020

git-svn-id: svn://svn.chromium.org/blink/trunk@157484 bbb929c8-8fbe-4397-9dbb-9b2b20218538