Don't try to schedule a GraphicsLayer flush when propagating changes to replicas inside a flush
https://bugs.webkit.org/show_bug.cgi?id=138532

Reviewed by Dan Bernstein.

When changing masks on replica layers with certain layer configurations,
it was possible to hit an assertion that a flush was being scheduled when already
flushing, via propagateLayerChangeToReplicas(). Fix by passing the scheduleFlush
flag along.

Not testable with current GraphicsLayer configurations.

* platform/graphics/ca/GraphicsLayerCA.cpp:
* platform/graphics/ca/GraphicsLayerCA.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@175781 268f45cc-cd09-0410-ab3c-d52691b4dbfc