RFR: 8260574: Remove parallel constructs in GenCollectedHeap::process_roots [v3]

[Albert Mingkun Yang]

> This PR is broken into two commits for easier reviewing, one for removing the usage of `SubTasksDone` in `GenCollectedHeap`, and the other for removing `StrongRootsScope` in the call chain of `GenCollectedHeap::process_roots`.
> 
> Tested: hotspot_gc

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  remove srs argument

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2280/files
  - new: https://git.openjdk.java.net/jdk/pull/2280/files/fbd1e5b6..9585038c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2280&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2280&range=01-02

  Stats: 8 lines in 3 files changed: 0 ins; 4 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2280.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2280/head:pull/2280

PR: https://git.openjdk.java.net/jdk/pull/2280