Instead of returning nullptr, just always call FatalProcessOutOfMemory
when we cannot allocate more memory.
In a follow-up CL, this should be extended to first try to run a GC and
see if this freed enough memory.
This CL is intentionally minimal in order to make it backmergable.

The unittest for WasmCodeManager needs to be refactored into a
parameterized test, such that each individual (parameterized) test can
die with OOM without affecting other tests.

R=mstarzinger@chromium.org

Bug: chromium:822266
Change-Id: I1336aa05ed50124b77ffaa4435ec9bed70e15c18
Reviewed-on: https://chromium-review.googlesource.com/966501Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52025}