Fix setting shape-outside: none

The CSS parser and CSSComputedStyleDeclaration use the none keyword for the
shape-outside property. CSSAnimatableValueFactory and StyleBuilderFunctions
treat the same shape value as the auto keyword.
The effect of this is that the none shape value is never applied.

According to the spec we should be using the none keyword.
http://dev.w3.org/csswg/css-shapes-1/#shape-outside-property

BUG=378071

Review URL: https://codereview.chromium.org/303473009

git-svn-id: svn://svn.chromium.org/blink/trunk@174936 bbb929c8-8fbe-4397-9dbb-9b2b20218538