RFR 8231999: Shenandoah: Traversal failed compiler/jsr292/CallSiteDepContextTest.java

[Zhengyu Gu]

This patch partially reverts JDK-8231293's fix, because it hides dead 
oops from GC, by returning NULL, which causes the failure of this test case.

The root cause of JDK-8231293 is that, Traversal deactivates SATB 
barrier too late, it should be turned off before weak root processing.

Bug: https://bugs.openjdk.java.net/browse/JDK-8231999
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8231999/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release) on Linux

After this fix, CallSiteDepContextTest.java test hangs in traversal 
mode, but it is separate issue, tracked by JDK-8232380.

Thanks,

-Zhengyu