RFR: 8260574: Remove parallel constructs in GenCollectedHeap::process_roots [v2]

[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:

  statically known sequential

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2280/files
  - new: https://git.openjdk.java.net/jdk/pull/2280/files/a13ce118..fbd1e5b6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2280&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2280&range=00-01

  Stats: 9 lines in 4 files changed: 3 ins; 0 del; 6 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