RFR: 8257072: ZGC: Rename roots iterators [v4]

[Per Liden]

On Thu, 26 Nov 2020 08:29:20 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> Per Liden has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Revert indentation change
>
> Marked as reviewed by stefank (Reviewer).

Thanks for reviewing!

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PR: https://git.openjdk.java.net/jdk/pull/1429