Reviewed by David Hyatt.

        Implement -webkit-linear-gradient and -webkit-radial-gradient
        https://bugs.webkit.org/show_bug.cgi?id=28152

        Add support for the parsing and rendering of non-repeating CSS3 linear gradients,
        according to <http://dev.w3.org/csswg/css3-images/#linear-gradients>.

        Tests: fast/gradients/css3-color-stop-units.html
               fast/gradients/css3-color-stops.html
               fast/gradients/css3-gradient-parsing.html
               fast/gradients/css3-linear-angle-gradients.html

        * css/CSSGradientValue.h:
        Add subclasses of CSSGradientValue for linear and radial gradients.

        * css/CSSGradientValue.cpp:
        (WebCore::CSSGradientValue::image): Add FIXME comment about why we can't cache
        the gradient sometimes.
        (WebCore::compareStops): positions are CSSValues now.
        (WebCore::CSSGradientValue::sortStopsIfNeeded): Add assertion that this is only called
        for old-style gradients.
        (WebCore::CSSGradientValue::addStops): New method that handles the spec rules about
        missing positions, ensuring that stop position is ordered, and spacing out stops
        without positions. Also normalize gradients which use stops outside the 0-1 range
        so that platforms can render them correctly.
        (WebCore::positionFromValue): Determin the position on the gradient line for percentage,
        number and length values.
        (WebCore::CSSGradientValue::computeEndPoint):
        (WebCore::CSSLinearGradientValue::cssText):
        (WebCore::endPointsFromAngle): Compute endpoints for a gradient which covers the rectangle
        given an angle.
        (WebCore::CSSLinearGradientValue::createGradient):
        (WebCore::CSSRadialGradientValue::cssText):
        (WebCore::CSSRadialGradientValue::resolveRadius):
        (WebCore::CSSRadialGradientValue::createGradient):

        * css/CSSParser.cpp:
        (WebCore::CSSParser::parseValue): Style fix.
        (WebCore::CSSParser::parseContent): Use isGeneratedImageValue() and parseGeneratedImage()
        (WebCore::CSSParser::parseFillImage): Use isGeneratedImageValue() and parseGeneratedImage()
        (WebCore::CSSParser::parseBorderImage): Use isGeneratedImageValue() and parseGeneratedImage()
        (WebCore::parseDeprecatedGradientPoint): Renamed to indicate that it's only used for -webkit-gradient.
        (WebCore::parseDeprecatedGradientColorStop): Renamed to indicate that it's only used for -webkit-gradient,
        and fixed to store CSSValues for the stop position.
        (WebCore::CSSParser::parseDeprecatedGradient): Renamed to indicate that it's only used for -webkit-gradient,
        and create CSSLinearGradientValue and CSSRadialGradientValue as appropriate.
        (WebCore::valueFromSideKeyword):
        (WebCore::parseGradientColorOrKeyword): Color parsing utility.
        (WebCore::CSSParser::parseLinearGradient): New method to parse linear gradients.
        (WebCore::CSSParser::parseRadialGradient): Stub for new method to parse radial gradients.
        (WebCore::CSSParser::isGeneratedImageValue): Detect gradients and CSS canvas.
        (WebCore::CSSParser::parseGeneratedImage): Parser for generated images like gradients
        and CSS canvas.
        * css/CSSParser.h:
        * platform/graphics/Gradient.h:
        (WebCore::Gradient::setP0): We need to be able to change the points when normalizing gradients.
        (WebCore::Gradient::setP1): Ditto.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@74912 268f45cc-cd09-0410-ab3c-d52691b4dbfc