Harden RenderBox::canBeScrolledAndHasScrollableArea logic https://bugs.webkit.org/show_bug.cgi?id=104373

Reviewed by Simon Fraser.
Patch by Antonio Gomes <a1.gomes@sisa.samsung.com>

Source/WebCore:

Previously if a say div has a 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.

Test: fast/events/autoscroll-overflow-hidden-longhands.html

WebKit autoscroll behavior now matches Firefox and Opera12 (pre-blink)
in that sense.

* rendering/RenderBox.cpp:
(WebCore::RenderBox::canBeProgramaticallyScrolled):
* rendering/RenderBox.h:
(WebCore::RenderBox::hasScrollableOverflowX):
(WebCore::RenderBox::hasScrollableOverflowY):

LayoutTests:

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

* fast/events/autoscroll-overflow-hidden-longhands-expected.txt: Added.
* fast/events/autoscroll-overflow-hidden-longhands.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154383 268f45cc-cd09-0410-ab3c-d52691b4dbfc