RFR: 8279700: Parallel: Simplify ScavengeRootsTask constructor API

[Albert Mingkun Yang]

Simple cleanup around `ScavengeRootsTask`.

Test: hotspot_gc

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Commit messages:
 - scavenge-old-gen-top

Changes: https://git.openjdk.java.net/jdk/pull/7011/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7011&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279700
  Stats: 23 lines in 1 file changed: 2 ins; 15 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7011.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7011/head:pull/7011

PR: https://git.openjdk.java.net/jdk/pull/7011