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

R=yangguo@chromium.org, rossberg@chromium.org
LOG=N
BUG=

Review URL: https://codereview.chromium.org/1136883006

Cr-Commit-Position: refs/heads/master@{#28458}