Remove runtimeEnabledShorthand flag in CSSShorthands.in

The runtimeEnabledShorthand flag currently has two uses; for animation
and text-decoration. In the case of text-decoration, the property is
used by editing as a non-shorthand property when the runtime flag is
off. For animations, we shouldn't get into the relevant function at all
when the flag is off. Removing this flag and having text-decoration as a
special hard-coded case simplifies the code.

Review URL: https://codereview.chromium.org/389933003

git-svn-id: svn://svn.chromium.org/blink/trunk@178162 bbb929c8-8fbe-4397-9dbb-9b2b20218538