Fix render process crash for windows debug builds on win10

Due to change 8ae5d7f0 in chromium DWriteFontProxy is now
also initialized when not using the sandbox. This triggers
on win10 DCHECK which fails if font fallback is supported,
but reported windows version (due to missing correct
manifest file) is 8.0. Skip the DCHECK for qtwebengine
builds.

Change-Id: I6fcef46b1e0f0dc8fec8fec62cc46322efcaaad9
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>