Fix macOS deployment target to match the one in Qt Core

The minimum required macOS deployment target in Qt 5.9 is 10.10. This
should be passed on to Chromium, rather than hardcoding the value to
10.7, which was used in pre-Qt 5.6 days to actually require a higher
version than Qt Core used at the time.

Task-number: QTBUG-60438
Change-Id: Id2ae47f467f8cc92403c1cd7a467335ebbba4e8e
Reviewed-by: Michael Brüning <michael.bruning@qt.io>