AX: Can't always increment web sliders.

https://bugs.webkit.org/show_bug.cgi?id=127451

Reviewed by Chris Fleizach.

Source/WebCore:

Clamping the decrement/increment amount to one when necessary (if a percent change would result in a change of less than one).

Test: accessibility/range-alter-by-percent.html

* accessibility/AccessibilityNodeObject.cpp:
(WebCore::AccessibilityNodeObject::changeValueByPercent):

LayoutTests:

Adding range-alter-by-percent.html to compliment the existing range-alter-by-step.html test.

* accessibility/range-alter-by-percent-expected.txt: Added.
* accessibility/range-alter-by-percent.html: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@162587 268f45cc-cd09-0410-ab3c-d52691b4dbfc