Implement runtime feature support for CSS Shorthands

Some newly implemented shorthand properties (eg. text-decoration) are 
only allowed if a runtime feature is enabled, with fallback to regular
non-shorthand parsing otherwise. This patch adds a 
'runtimeEnabledShorthand' optional variable that must contain the name
of the runtime feature (eg. CSS3TextDecorations).

This fixes an issue where an assertion in StyleResolver indicated that
it expected a shorthand property (see link below for details):
https://codereview.chromium.org/20262002/#msg28

BUG=165462

Review URL: https://chromiumcodereview.appspot.com/22610005

git-svn-id: svn://svn.chromium.org/blink/trunk@155866 bbb929c8-8fbe-4397-9dbb-9b2b20218538