The button is drawn with:
EditorGUILayout.EditorToolbarForTarget(EditorGUIUtility.TrTempContent("Edit Collider"), this);
See the source here.
If you just want the GUIContent, then that’s the TrTempContent part.
Well, it looks like that the TrTempContent method is just a way to create some sort of pooled GUIContent. So this method just creates the text “Edit Collider” for the tool bar. The “EditorToolbarForTarget” method seems to automatically fill itself with the available EditorTools for the object you pass in, in this case for a BoxCollider2D.
The relevant EditorTool is defined in the same file just below the custom editor. The toolbar icon is defined in another class called PrimitiveBoundsHandle. However “editModeButton” is an internal property so we can not use it directly. Though the property creates the GUIContent for the icon like this.
So this should give you the image of the button:
EditorGUIUtility.IconContent("EditCollider").image