* gc.c: fix major GC flags.
  * add GPR_FLAG_MAJOR_BY_FORCE, which indicates
    major GC by METHOD, CAPI and so on (see GC_BY).
  * remove GPR_FLAG_MAJOR_BY_RESCAN because not used.
  * remove GPR_FLAG_MAJOR_BY_STRESS, use FORCE instead.
* test/ruby/test_gc.rb: catch up.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46927 b2dd03c8-39d4-4d8f-98ff-823fe69b080e