RFR: 8264279: Shenandoah: Missing handshake after JDK-8263427

[Zhengyu Gu]

JDK-8263427 mistakenly removed handshake after concurrent weak roots, which is needed to flush out stale oops before we can safely recycle trash regions. Otherwise, those stale oops are suddenly resurrected, because they are above TAMS after regions are recycled.

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - JDK-8264279

Changes: https://git.openjdk.java.net/jdk/pull/3248/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3248&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264279
  Stats: 29 lines in 5 files changed: 20 ins; 8 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3248.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3248/head:pull/3248

PR: https://git.openjdk.java.net/jdk/pull/3248