RFR: Make ResourceMark use consistent in ShenandoahCodeRoots

[Roman Kennke]

During last merge, I added ResourceMark in remove_nmethod() to guard the
GrowableArray allocation in ShenandoahNMethodOopDetector. This guy is
also used in add_nmethod() and shall need similar protection. Let's put
it right in ShenandoahNMethodOopDetector to make it consistent.

When backporting, ignore the removed ResourceMark in remove_nmethod().

Testing: tier3_gc_shenandoah

http://cr.openjdk.java.net/~rkennke/coderoot-resourcemark/webrev.00/

Ok?

Roman