RFR(T) 8229231: Shenandoah: Non-PCH builds failed after JDK-8224932

[Zhengyu Gu]

Please review this trivial patch that fixes breakage of Shenandoah/jdk 
Non-PCH builds.

I could not reproduce on jdk/jdk, so let's push to shenandoah/jdk.

Bug: https://bugs.openjdk.java.net/browse/JDK-8229231
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8229231/webrev.00/

Test:
   Non-PCH build on Windows.

Thanks,

-Zhengyu