RFR (11u, S): backport JDK-8247615 Initialize the bytes left for the heap sampler

[Jean Christophe Beyler]

Hi all,

First time I request a backport so if I am missing anything, please let me
know :)

I am trying to port this webrev into 11u:

Summary:
This fixes a sampling bias for the first allocations and ensures that the
sampling rate is correct from the start.
It is safe to apply as the whole code is protected behind flags and only is
used when JVMTI enables the heap monitoring.
We added a test that tickles this code on purpose.

Porting issues:
Finally, the patch does not apply cleanly due to slight differences with
the files. This webrev provides the clean to JDK11u:
http://cr.openjdk.java.net/~jcbeyler/8247615/webrev.02_11/

- That webrev compiles and tests for HeapMonitor (with the new test) pass:
    make run-test TEST="test/hotspot/jtreg/serviceability/jvmti/HeapMonitor"

compared to what I submitted to head:
http://cr.openjdk.java.net/~jcbeyler/8247615/webrev.02/

Let me know what else I can do or provide!

Thanks for all your help,
Jc

Ps: I sent this email before I subscribed so one is waiting in moderation;
if this shows up in double, I apologize :)