[8u] RFR for backport of 8156028: G1YoungGenSizer _adaptive_size not correct when setting NewSize and MaxNewSize to the same value

[Liang Mao]

Hi,
Original bug:
 https://bugs.openjdk.java.net/browse/JDK-8156028
 http://hg.openjdk.java.net/jdk/jdk/rev/ba8be1a71dec

Original patch does not apply cleanly to 8u, because class G1YoungGenSizer has been
moved to seperated file.

Since G1 is already a stable feature in JDK8U, the obvious bug that the young gen size
cannot be adaptive with different NewSize and MaxNewSize makes users confused. The
alternative way to use experimental option *G1NewSizePercent* is not intuitive and formal.

8u webrev:
 http://cr.openjdk.java.net/~ddong/8156028/webrev.00/
Testing: x86_64 build, affected tests, tier1
Thanks,
Liang