port d6ee366d (r29834).

original commit message:

    This is the initial round of optimizations for the
    LoadGlobalViaContextStub and StoreGlobalViaContextStub, basically
    turning them into platform code stubs to avoid the Crankshaft overhead
    in the fast case, and making the runtime interface cheaper.

BUG=

Review URL: https://codereview.chromium.org/1258513003

Cr-Commit-Position: refs/heads/master@{#29839}