Fix three bugs in the equals() implementations for css gradients.

https://bugs.webkit.org/show_bug.cgi?id=122987

Reviewed by Andreas Kling.

1. Linear gradients were considered equal if the first gradient has no x and y
   position and the second has no x but does have y.
2. Same as 1, for radial gradients. (This doesn't happen in practice as
   CSSParser::parseRadialGradient rejects such input, so no test for this case.)
3. Radial gradients without x and y position weren't considered equal even if
   they were.

Source/WebCore:

* css/CSSGradientValue.cpp:
(WebCore::CSSLinearGradientValue::equals):
(WebCore::CSSRadialGradientValue::equals):

LayoutTests:

Merges https://codereview.chromium.org/26147006/

* cssom/cssvalue-comparison-expected.txt:
* cssom/cssvalue-comparison.html:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@157598 268f45cc-cd09-0410-ab3c-d52691b4dbfc