RFR: 8247281: migrate ObjectMonitor::_object to OopStorage [v5]

[Daniel D.Daugherty]

On Wed, 16 Sep 2020 08:23:03 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

>> Marked as reviewed by sspitsyn (Reviewer).
>
> I added a release note (https://bugs.openjdk.java.net/browse/JDK-8253225) describing that these roots are now weak, and
> hence won't be reported. Please have a look at that, to make sure what I am describing makes sense.

@kimbarrett - Thanks for the cleanup suggestion.
@sspitsyn - Thanks for the review.
I've gone ahead and "resolved" the comments that were addressed by previous commits.
I've made changes for both Kim and Serguei's reviews. Building those bits now.

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PR: https://git.openjdk.java.net/jdk/pull/135