This CL adds general lazy compilation support to WebAssembly, according
to the design described in the design doc (see referenced bug).

It's not used currently, but I tested locally that all tests succeed if
I enable it by default.

With a later CL, we will enable lazy compilation by default for
validate-asm: https://chromium-review.googlesource.com/451318

R=titzer@chromium.org, ahaas@chromium.org, bmeurer@chromium.org
BUG=v8:5991

Change-Id: I85440382118a24fc245e78a5a90cf2b95659cd69
Reviewed-on: https://chromium-review.googlesource.com/451317
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43936}