The wasm context will always be stored on the stack, and will be loaded
from there whenever needed (for accessing globals or the memory). We
can still improve this later by caching the context address or specific
information loaded from it.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Idd7fb1ccff28a73beaf545997e3dfdb74757b686
Reviewed-on: https://chromium-review.googlesource.com/779145
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49540}