8034761: Remove the do_code_roots parameter from process_strong_roots

[Stefan Karlsson]

Hi all,

Please, review this patch to remove the do_code_roots parameter from 
SharedHeap::process_strong_roots.

The changes done are:
- Change the code to rely on the ScannningOption so parameter instead of 
do_code_roots.
- Change GenMarkSweep and G1MarkSweep to adjust the code roots with the 
help of process_strong_roots instead of doing it as a separate phase 
after process_strong_roots.
- Removed the unused FalseClosure.

After this patch the adjust phase of the GenMarkSweep and G1MarkSweep 
will use the generic code in process_strong_roots, which mark/claim the 
nmethods before they are processed. Before the patch these two Serial 
Old GC adjust phases skipped the mark/claim part. No noticeable Serial 
Old GC time increases were found when this patch was performance tested.

This cleanup is needed/wanted for G1 Class Unloading.

Webrev:
http://cr.openjdk.java.net/~stefank/8034761/webrev.00/

RFE:
https://bugs.openjdk.java.net/browse/JDK-8034761

thanks,
StefanK
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