Fix for scrolling with modal opened. (#12899)

Change-Id: I9ba142a35ab887bb3a71c7311064ebab77bc01ba
Merge commit 'fbc48c6ab75479560919a463ba5693b44bb0e811' into merge71

Conflicts:
	client/src/com/vaadin/client/ui/VWindow.java

Change-Id: I6b4605ffafe990747757ec545cf2b52656c9f9f4