[stubs] Optimize LoadGlobalViaContextStub and StoreGlobalViaContextStub.

[stubs] Optimize LoadGlobalViaContextStub and StoreGlobalViaContextStub.

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.

R=ishell@chromium.org
BUG=chromium:510694
LOG=n

Review URL: https://codereview.chromium.org/1238143002

Cr-Commit-Position: refs/heads/master@{#29834}