This code depends on the QtWebEngine (not Qt) version, so it should actually check for that.
Otherwise it breaks if a distribution ships Qt 5.9 LTS together with a newer QtWebEngine, like e.g. openSUSE Leap 15.0 does, or QtWebEngine 5.9 together with a newer Qt as is the case in FreeBSD.

BUG: 397349
CCBUG: 388440

Compiled messagelib with the following combinations:

1. Qt 5.9.4/QtWebEngine 5.9.4
2. Qt 5.11.2/QtWebEngine 5.11.2
3. Qt 5.9.4/QtWebEngine 5.10.1

In each case, the intended code path is taken now, i.e. it returns document.body.scrollTop in the first case document.documentElement.scrollTop for the others.

Also tested it on a real openSUSE Leap 15.0 installation (which comes with Qt 5.9.4 and QtWebEngine 5.10.1), PgUp/PgDown works fine now, it only worked once before.