port d211608a

original commit message:
  [turbofan] Remove the no-context hack for JSToNumber.

  The ToNumberStub is now able to handle all plain primitives (Numbers,
  Booleans, Null, Undefined and Strings) without context access.

BUG=

Review URL: https://codereview.chromium.org/810683002

Cr-Commit-Position: refs/heads/master@{#25833}