getPropertyValue for -webkit-margin-collapse returns null, should compute the shortha...

getPropertyValue for -webkit-margin-collapse returns null, should compute the shorthand value
https://bugs.webkit.org/show_bug.cgi?id=110903

Patch by Uday Kiran <udaykiran@motorola.com> on 2013-02-26
Reviewed by Alexander Pavlov.

Source/WebCore:

getPropertyValue("-webkit-margin-collapse") should compute shorthand value
when -webkit-margin-collapse is specified in CSS.

Test: fast/css/getPropertyValue-webkit-margin-collapse.html

* css/StylePropertySet.cpp:
(WebCore::StylePropertySet::getPropertyValue):

LayoutTests:

Added test for getPropertyValue("-webkit-margin-collapse").

* fast/css/getPropertyValue-webkit-margin-collapse-expected.txt: Added.
* fast/css/getPropertyValue-webkit-margin-collapse.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@144145 268f45cc-cd09-0410-ab3c-d52691b4dbfc