During WebAssembly compilation and instantiation we entered a
{CodeSpaceMemoryModificationScope} several times per function. This
introduced significant overhead, see the referenced bug. With this CL
we enter the {CodeSpaceMemoryModificationScope} on a per-module
granularity and not on a function granularity. We enter now the
following scopes:

* one scope for the whole synchronous compilation;
* one scope for each finishing step in asynchronous compilation (each
    step finishes multiple functions);
* one scope for module instantiation, without the execution of the
  start function.

Locally these changes reduced the overhead significantly.

R=mstarzinger@chromium.org, titzer@chromium.org
CC=clemensh@chromium.org

Bug: chromium:787731
Change-Id: I5c5694544a97f4c1e5a2a29da9a005d0ca7616bd
Reviewed-on: https://chromium-review.googlesource.com/787851Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49618}