This reverts commit 1eb1dfab.

The original compilation separation change avoided associating a heap
for the wasm instance if memory was not provided, nor needed. The
grow memory CL assumed the old behavior, where a memory buffer was
always present, but may have had a zero size.

The 2CLS  landed shortly after one another. We decided to treat the
grow memory as the race condition winner, so this CL here re-lands
compilation separation, plus adjusts grow memory to deal with
the undefined mem buffer.

BUG=

Review-Url: https://codereview.chromium.org/2102193003
Cr-Commit-Position: refs/heads/master@{#37352}