port 47502a23 (r32381)

  original commit message:
  Previously all contexts had a link to the global object, but what is
  required in most cases (except for the global load, store and delete
  case) is the native context.

  This also removes the second dummy global object that was still linked
  to every native context. We will add a different mechanism to ensure
  that builtins do not pollute the actual global object during
  bootstrapping.

  Drive-by-fix: Unify some MacroAssembler magic and drop obsolete stuff.

BUG=

Review URL: https://codereview.chromium.org/1481353002

Cr-Commit-Position: refs/heads/master@{#32387}