Answer: the shared electrons are closer to cl due to the greater electronegativity of chlorine.
Justification:
1) In a covalent bond (the bond between two non-metal elements) the electrons are shared.
2) When the two elements have different electronegativities the electrons are not evely shared: the element with the greatest electronegativity will pull the electrons with higher strength.
3) As consequence the electrons will be closer to the most electronegative atom creating a dipole moment, which is what causes the molecule to be polar.
4) You can learn the trend of electronegativity in the periodic table: in a group the electronegativity decreases downward. With this you do not need a table with the values of the electronegativities and can predict that the electronegativity of Cl is greater than the electronegativity of Br.
5) From above statements you can conclude that, due to its greater electronegativity, chorine will pull the electrons with greater strength, causing the molecule to be polar.