[css3-text]  Implement text-decoration property shorthand

Adds support for CSS Level 3 text-decoration property implementation as
shorthand (enabled via runtime flag), according to the specification:

http://www.w3.org/TR/css-text-decor-3/#text-decoration-property

Backported from WebKit: https://bugs.webkit.org/show_bug.cgi?id=92000

All layout tests that verifies/uses text-decoration computed style were
updated. Editing behavior is preserved for text-decoration.

BUG=165462
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/20262002

git-svn-id: svn://svn.chromium.org/blink/trunk@155719 bbb929c8-8fbe-4397-9dbb-9b2b20218538