qtwebengine's chromium build uses all core despite using 'make -j$N' from the top level

Hello,

For a typical source build of Qt using:

```

./configure \
      -opensource \
      -confirm-license \
      -nomake examples \
      -nomake tests
make -j2

```

The chromium built during qtwebengine's build would end up using all cores and fail to link due to exhausting the system's memory (32 GiB) instead of honoring the intent of limiting the build to use two parallel processes (-j2).

This is because Chromium's build system doesn't get the information from the top level correctly. It should call ninja with the same -j argument as the top level make. One workaround is to export NINJAFLAGS=-j2 before calling make at the top level, or pass it as an argument (for GNU Make).

Thanks!