REGRESSION (r166860): ASSERTION FAILED: !isCalculated() on fast/css/image-set-value-not-removed-crash.html
https://bugs.webkit.org/show_bug.cgi?id=131480

Reviewed by Andreas Kling.

Source/WebCore:

Fixes intermittent assertion failure in fast/css/image-set-value-not-removed-crash.html.

* css/CSSComputedStyleDeclaration.cpp:
(WebCore::valueForImageSliceSide): Added. Helper used below in valueForNinePieceImageSlice.
Handles calculated values by returning 0; incorrect but predictable.
(WebCore::valueForNinePieceImageSlice): Updated to call valueForImageSliceSide.
(WebCore::positionOffsetValue): Use nullptr.
(WebCore::ComputedStyleExtractor::propertyValue): Updated to call positionOffsetValue
by its new name. Removed "get" from the name.
(WebCore::positionOffsetValue): Renamed from getPositionOffsetValue.

* platform/Length.h: Made isCalculated public.

LayoutTests:

* platform/mac/TestExpectations: Unskip the test now that the assertion is fixed.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@167192 268f45cc-cd09-0410-ab3c-d52691b4dbfc