Rename isExpandedShorthand() to isShorthandProperty()

This patch renames isExpandedShorthand() to isShorthandProperty().
I and others have found this naming confusing when dealing with shorthands.
The name seems to suggest returning true for longhand properties that may
have originated from shorthand properties, it actually returns whether
the property is a shorthand property.

Review URL: https://codereview.chromium.org/194243003

git-svn-id: svn://svn.chromium.org/blink/trunk@184614 bbb929c8-8fbe-4397-9dbb-9b2b20218538