RFR 8158237: JVMTI hides critical debug information for memory leak tracing

[Coleen Phillimore]

Summary: remove _backtrace as hidden field, since the original problem 
no longer exists

The backtrace format is changed to be all java objects and no longer 
have jvm methodOops (pseudo Objects) which caused the crash.

This change is being also tested by the customer.  I ran full nightly 
tests on this.

bug link https://bugs.openjdk.java.net/browse/JDK-8158237
open webrev at http://cr.openjdk.java.net/~coleenp/8158237.01/webrev
open webrev at http://cr.openjdk.java.net/~coleenp/8158237.jdk.01/webrev

Thanks,
Coleen