port 1efc1e4f (r28458).

original commit message:

  This reapplies https://codereview.chromium.org/1136073002, along with
    the followups:

      Remove Scope::scope_uses_this_ flag
      https://codereview.chromium.org/1128963005

    and

      PPC: Resolve references to "this" the same way as normal variables
      https://codereview.chromium.org/1134073003

BUG=

Review URL: https://codereview.chromium.org/1135233003

Cr-Commit-Position: refs/heads/master@{#28469}