Web Animations CSS: Support animation of flex-grow, flex-shrink and flex-basis

Animation of these properties is defined by css-flexbox. Note especially the
non-continuous behavior when animating flex-grow/shrink to/from zero.

Although these properties were originally intended to be animatable in the
existing engine, support was broken when -webkit-flex became a shorthand.

BUG=257591,301348

Review URL: https://codereview.chromium.org/25202002

git-svn-id: svn://svn.chromium.org/blink/trunk@159095 bbb929c8-8fbe-4397-9dbb-9b2b20218538