A fix would be to walk the reloc info and RecordWriteIntoCode. Doing
so, however, upsets a scavenger DCHECK.

We stumbled upon this issue because we were placing wasm objects
(fixed arrays) in NEW_SPACE, rather than OLD_SPACE. These fixed
arrays were subsequently referenced from Code objects, which were
then cloned.

The current CL ensures wasm constructs are allocated in OLD_SPACE,
by pre-tenuring them (consistent with other wasm allocations). In
addition, it adds a DCHECK for CopyCode clarifying its lack of support
for references to NEW_SPACE.

We can investigate in a subsequent CL making CopyCode more robust,
pending understanding of the Scavenger's assumptions.

BUG=

Review-Url: https://codereview.chromium.org/2201663003
Cr-Commit-Position: refs/heads/master@{#38263}