Implement CSS clip property in CSSStyleApplyProperty.

Implement CSS clip property in CSSStyleApplyProperty.
https://bugs.webkit.org/show_bug.cgi?id=74913

Reviewed by Andreas Kling.

No new tests / refactoring only.

* css/CSSPrimitiveValue.h:
* css/CSSPrimitiveValueMappings.h:
(WebCore::CSSPrimitiveValue::convertToLength):
This new function aims to provide a single call for converting many CSSPrimitiveValue
values to Lengths. It is templated to allow the caller to specify which conversions
are appropriate depending on the context in which the value is used.
* css/CSSStyleApplyProperty.cpp:
(WebCore::ApplyPropertyClip::convertToLength):
(WebCore::ApplyPropertyClip::applyInheritValue):
(WebCore::ApplyPropertyClip::applyInitialValue):
(WebCore::ApplyPropertyClip::applyValue):
(WebCore::ApplyPropertyClip::createHandler):
(WebCore::CSSStyleApplyProperty::CSSStyleApplyProperty):
* css/CSSStyleSelector.cpp:
(WebCore::CSSStyleSelector::applyProperty):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@105693 268f45cc-cd09-0410-ab3c-d52691b4dbfc