Implements support for declaring global variables. Also adds support for loading
from and storing to both global and unallocated global variables.  Adds the
following bytecodes:
 - StoreGlobal
 - LoadContextSlot

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1378523005

Cr-Commit-Position: refs/heads/master@{#31166}