Who're.
Contractions are always informal - they are governed by rules no more strict than slang or colloquialisms. In fact, some contractions are colloquial. To claim that a contraction is unrecognised, especially one that is often used in spoken English, would be arrogant.
Other contractions include:
you're
we're
they're
Remember, the English language has its own set of rules and there are so many exceptions, the only important thing to do is memorize the correct spellings and word forms.
The contraction "who're" is short for "who are".
Ther is no contraction for who was
who'll
who's
There is no contraction. The contraction we're means "we are."
There is no contraction for I was. There is a contraction for I am (I'm) and for I have (I've).
There is no contraction of we'll. It is a contraction, meaning "we will."
There is no contraction for 'pram', it is a contraction of the word perambulator.
There is no contraction. The word isn't is the contraction, meaning "is not."
There is no contraction for were you. There is no contraction for you were.There is a contraction for "you are" (you're).
There is no contraction for "its not."There is a contraction for "it is" (it's).There is a contraction for "is not" (isn't).
He's already IS a contraction. It is a contraction of he is.
There is no contraction. There is a contraction doesn't which means "does not."
There is no contraction. The contraction we're means "we are."
There is no contraction for I was. There is a contraction for I am (I'm) and for I have (I've).
The contraction for "were" is "we're."
It's is the contraction of it has and it is.
The contraction for "he will" is "he'll."
"They've" is the contraction for 'they' and 'have'.
Didn't is a contraction of the words "did" and "not". There is no 'contraction' for this word.
The grammatical contraction for "have not" is "haven't."