[ComboBox] Initial size and size after making empty are not equal.

See attached movie (I added 1 item and removed it) You can see, that initial width and final width are not equal.

Also, about size, i may note, (according to testcase failures analisys) that it is good, when size of empty control is stable, (before and after some operation it is not changed, if in the beginning and in the end list of items stays empty). Also, you may look on RT-18052.