Generate toCSSFooValue() for CSSFooValues

As a step of generating toCSSFooValue(), this cl adds toCSSFooValue() for some CSS child values. (CSSCanvasValue, CSSCrossfadeValue, CSSImageGeneratorValue, CSSImageValue, CSSInitialValue, CSSTimingFunctionValue, CSSCubicBezierTimingFunctionValue and CSSStepsTimingFunctionValue)

This will help to detect bad type casts. To support some toCSSFooValue(), isCanvasValue(), isUnicodeRangeValue(), and isCrossfadeValue() are addeded to CSSValue.h.

BUG=N/A

Review URL: https://codereview.chromium.org/25434004

git-svn-id: svn://svn.chromium.org/blink/trunk@158916 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Web Animations CSS: Handle -webkit-column-count property

BUG=240653
R=dstockwell

Review URL: https://codereview.chromium.org/25565003

git-svn-id: svn://svn.chromium.org/blink/trunk@158917 bbb929c8-8fbe-4397-9dbb-9b2b20218538