Only default to transition shorthand if all 4 values are set

When setting any individual transition property the
transition shorthand was used for cssText. This patch
changes the behavior so that only when all 4 individual
properties have a value set, the shorthand is used. A
test is added to verify this.

Matches Firefox behavior.

BUG=310403

Review URL: https://codereview.chromium.org/617493002

git-svn-id: svn://svn.chromium.org/blink/trunk@183024 bbb929c8-8fbe-4397-9dbb-9b2b20218538