This avoids unnecessary stack-walks to determine the current context in
WebAssembly runtime functions, in cases where the calling stub already
determined the calling instance and can just set the context register
itself before calling into the runtime.

R=clemensh@chromium.org

Change-Id: Iba02d479a7dad8907195bf94efb9d559be20a6d1
Reviewed-on: https://chromium-review.googlesource.com/1228035Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55962}