overflow-y: hidden areas can still be scrolled vertically

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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=157484

[eseidel, 2013-09-09 19:20:13]

lgtm

[commit-bot: I haz the power, 2013-09-09 19:20:25]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/a1.gomes@sisa.samsung.com/23567020/3001

[commit-bot: I haz the power, 2013-09-09 21:17:36]

Retried try job too often on win_blink_rel for step(s) webkit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_blink_...

[commit-bot: I haz the power, 2013-09-09 22:41:42]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/a1.gomes@sisa.samsung.com/23567020/3001

[commit-bot: I haz the power, 2013-09-09 23:44:03]

Change committed as 157484