When we enter a method that needs access to the [[HomeObject]]
we allocate a local variable `.home_object` and assign it the
value from the [[HomeObject]] private symbol. Something along
the lines of:

  method() {
    var .home_object = %ThisFunction()[home_object_symbol];
    ...
  }

BUG=v8:3867, v8:4031
LOG=N

Review URL: https://codereview.chromium.org/1135243004

Cr-Commit-Position: refs/heads/master@{#28644}