Add animatable flag to CSSProperties.in

This patch adds the flag 'animatable' to CSSProperties.in, replacing the
existing switch statement in CSSAnimations.cpp. Setting the flag
indicates that the property can be animated by CSS animations and
transitions.

The added code will generate a function in CSSPropertyMetadata, which
will eventually also support functions like isInheritedProperty and
isValidFirstLetterStyleProperty. This means that the individual lines in
CSSProperties.in may start getting a bit long, although a centralized
specification of our supported CSS properties arguably helps developers
add new CSS properties, by reducing the lines they need to add and
adding documentation on what changes need to be made.

BUG=396992

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=180039

Review URL: https://codereview.chromium.org/454423003

git-svn-id: svn://svn.chromium.org/blink/trunk@180213 bbb929c8-8fbe-4397-9dbb-9b2b20218538