Allow use of ordinary shared array buffer in wasm threads

For wasm memory buffers, we normally require the memory to be allocated
via WebAssembly.Memory, which will set a is_wasm_memory flag and
register the memory with the wasm allocation tracker. This CL weakens
that requirement in a DCHECK to allow for running experimental threaded
applications even though the is_wasm_memory flag is not currently
propagated via postMessage.

Bug: chromium:836800
Change-Id: I4613b8651423307ce4cd466c0df28fc43244ec4f
Reviewed-on: https://chromium-review.googlesource.com/1027813
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52801}

src/wasm/module-compiler.cc

@@ -1753,9 +1753,12 @@ MaybeHandle<WasmInstanceObject> InstanceBuilder::Build() {
     Handle<JSArrayBuffer> memory = memory_.ToHandleChecked();
     memory->set_is_neuterable(false);
 
-    DCHECK_IMPLIES(use_trap_handler(), module_->is_asm_js() ||
-                                           memory->is_wasm_memory() ||
-                                           memory->backing_store() == nullptr);
+    DCHECK_IMPLIES(use_trap_handler(),
+                   module_->is_asm_js() || memory->is_wasm_memory() ||
+                       memory->backing_store() == nullptr ||
+                       // TODO(836800) Remove once is_wasm_memory transfers over
+                       // post-message.
+                       (FLAG_experimental_wasm_threads && memory->is_shared()));
   } else if (initial_pages > 0 || use_trap_handler()) {
     // We need to unconditionally create a guard region if using trap handlers,
     // even when the size is zero to prevent null-derefence issues