Remove superfluous CSSLinearTimingFunctionValue

We can simply use CSSValueLinear, as we do for CSSValueEase etc.

No change in behavior.

R=dstockwell

Review URL: https://chromiumcodereview.appspot.com/23678004

git-svn-id: svn://svn.chromium.org/blink/trunk@157051 bbb929c8-8fbe-4397-9dbb-9b2b20218538