G1YoungGenSizer ignores these values if they are not from the command line. Resulting in a different _sizer_kind than expected.
The NewSize and MaxNewSize are also not bounded by the Initial and Max heap size respectively (JDK-8370494).

I suggest we rewrite this logic using `!FLAG_IS_DEFAULT` to always get the correct behaviour. And only use `FLAG_IS_CMDLINE` for issuing warnings to the user when we ergonomically change or ignore these values to be compatible.
As well as adding the proper truncation for HeapSize incompatible values.