Re-enable building with gcc 4.9.1

The code that gcc 4.9.1 stumpled on was rewritten in Chromium 56, so
we should try and let compiler build webengine again.

Change-Id: I9ccec7e93aaa384aefdf338e3822b78e0dc494f2
Reviewed-by: Michal Klocek <michal.klocek@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>