This CL refactors the WasmMemoryObject and WasmInstanceObject classes to
use WeakFixedArray instead of using a doubly-linked list of instances. This
simplifies the lifetime management of instances by not requiring them to
be unlinked from this list upon GC. It also simplifies the iteration over
the instances using a given WasmMemoryObject.

Note that, contrary to my naive assumption at the outset, it is still necessary for the InstanceFinalizer (called upon a WasmInstanceObject death) to unlink itself from a WasmMemoryObject's instances list, due to finalizer ordering.

R=deepti@chromium.org, mlippautz@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2972803002
Cr-Commit-Position: refs/heads/master@{#46482}