RFR: 8295012: Arena should not derive from CHeapObj<mtNone> [v2]

[Kim Barrett]

> Please review this change to the `Arena` class.  Rather than deriving from
> `CHeapObj<mtNone>` (which has the effect of preventing C-heap allocation for an
> arena by asserting if such is attempted), instead have no allocation base
> class so that `new Arena(...)` will fail to compile.
> 
> Testing: mach5 tier1

Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:

  Arena derives from CHeapObjBase

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10621/files
  - new: https://git.openjdk.org/jdk/pull/10621/files/1e64a3cd..828541f5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10621&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10621&range=00-01

  Stats: 33 lines in 2 files changed: 0 ins; 32 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10621.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10621/head:pull/10621

PR: https://git.openjdk.org/jdk/pull/10621