As of Chrome 52, the UI has changed. When the Developer Tools dialog is open, you select the vertical ellipsis and can then choose the docking position:
Select the icon on the left to open the Chrome Developer Tools in a new window: 
Previously
Click and hold the button next to the close button of the Developer Tool in order to reveal the “Undock into separate window” option.
Note: A “press” is not enough in that state.