The WasmMemoryTracker keeps track of reserved memory in order to avoid
running out of virtual address space. So far, we were only tracking
reservations for wasm memory, and not for code. This CL changes that to
also include code reservations.

Drive-by: Some cleanup around the allocation of the WasmCodeManager.

R=titzer@chromium.org

Bug: chromium:883639
Change-Id: I0c2586a742022ae00752132e048346d54e2a1a7c
Reviewed-on: https://chromium-review.googlesource.com/1230134Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56001}