RFR: 8280932: G1: Rename HeapRegionRemSet::_code_roots accessors

[Albert Mingkun Yang]

Mostly mechanical changes of renaming some methods/comments.

Test: build

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

Commit messages:
 - rename-strong-code-roots

Changes: https://git.openjdk.java.net/jdk/pull/7285/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7285&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280932
  Stats: 93 lines in 13 files changed: 0 ins; 1 del; 92 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7285.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7285/head:pull/7285

PR: https://git.openjdk.java.net/jdk/pull/7285