Don't UseCounter CSS properties on recursive parseValue calls

Currently we count CSS properties in CSSPropertyParser::parseValue (the
non-static version). This function is used recursively for shorthands,
so we end up tracking many CSS properties which haven't been specified
by users.

This patch moves the check up a level, so that it is only called on
properties specified by a user. I've also made it only count properties
that succeeded at parsing (behaviour wouldn't change for declarations
that fail to parse if we removed the property).

This should reduce the usage numbers on chromestatus.com for many
properties. For example while border-spacing is not specced as a
shorthand, it is implemented as a shorthand and we end up measuring
-webkit-border-horizontal-spacing whenever we parse border-spacing.

Review URL: https://codereview.chromium.org/642583006

git-svn-id: svn://svn.chromium.org/blink/trunk@183636 bbb929c8-8fbe-4397-9dbb-9b2b20218538