hg: openjfx/8u/rt: 8204856: WebEngine document becomes null after PAGE_REPLACED event

[kevin.rushforth at oracle.com]

Changeset: 911a239bb117
Author:    arajkumar
Date:      2018-07-12 10:16 +0530
URL:       http://hg.openjdk.java.net/openjfx/8u/rt/rev/911a239bb117

8204856: WebEngine document becomes null after PAGE_REPLACED event
Reviewed-by: mbilla, kcr

! modules/web/src/main/java/javafx/scene/web/WebEngine.java
! modules/web/src/test/java/javafx/scene/web/HistoryStateTest.java